Add minimal ASN.1 encoding/decoding primitives.

Add ASN.1 DER serialization for EC NIST curves.

libp2p/crypto/ecnist.nim

@@ -8,6 +8,7 @@
 ## those terms.
 import common
 import nimcrypto/utils
+import minasn1
 
 const
   PubKey256Length* = 65
@@ -120,24 +121,6 @@ proc getOffset(seckey: EcPrivateKey): int {.inline.} =
   else:
     result = cast[int](o)
 
-proc copyKey(dest: var openarray[byte], seckey: EcPrivateKey): bool {.inline.} =
-  let length = seckey.key.xlen
-  if length > 0:
-    if len(dest) >= length:
-      let offset = getOffset(seckey)
-      if offset >= 0:
-        copyMem(addr dest[0], unsafeAddr seckey.buffer[offset], length - offset)
-        result = true
-
-proc copyKey(dest: var openarray[byte], pubkey: EcPublicKey): bool {.inline.} =
-  let length = pubkey.key.qlen
-  if length > 0:
-    if len(dest) >= length:
-      let offset = getOffset(pubkey)
-      if offset >= 0:
-        copyMem(addr dest[0], unsafeAddr pubkey.buffer[offset], length - offset)
-        result = true
-
 template getSignatureLength*(curve: EcCurveKind): int =
   case curve
   of Secp256r1:
@@ -313,57 +296,92 @@ proc `$`*(sig: EcSignature): string =
     else:
       result = toHex(sig.buffer)
 
-proc toBytes*(seckey: EcPrivateKey, data: var openarray[byte]): bool =
+proc toBytes*(seckey: EcPrivateKey, data: var openarray[byte]): int =
-  ## Serialize EC private key ``seckey`` to raw binary form and store it to
+  ## Serialize EC private key ``seckey`` to ASN.1 DER binary form and store it
-  ## ``data``.
+  ## to ``data``.
   ##
-  ## If ``seckey`` curve is ``Secp256r1`` length of ``data`` array must be at
+  ## Procedure returns number of bytes (octets) needed to store EC private key,
-  ## least ``SecKey256Length``.
+  ## or `0` if private key is not in supported curve.
-  ##
-  ## If ``seckey`` curve is ``Secp384r1`` length of ``data`` array must be at
-  ## least ``SecKey384Length``.
-  ##
-  ## If ``seckey`` curve is ``Secp521r1`` length of ``data`` array must be at
-  ## least ``SecKey521Length``.
-  ##
-  ## Procedure returns ``true`` if serialization successfull, ``false``
-  ## otherwise.
   if seckey.key.curve in EcSupportedCurvesCint:
-    if copyKey(data, seckey):
+    var offset, length: int
-      result = true
+    var pubkey = seckey.getKey()
+    var b = Asn1Buffer.init()
+    var p = Asn1Composite.init(Asn1Tag.Sequence)
+    var c0 = Asn1Composite.init(0)
+    var c1 = Asn1Composite.init(1)
+    if seckey.key.curve == BR_EC_SECP256R1:
+      c0.write(Asn1Tag.Oid, Asn1OidSecp256r1)
+    elif seckey.key.curve == BR_EC_SECP384R1:
+      c0.write(Asn1Tag.Oid, Asn1OidSecp384r1)
+    elif seckey.key.curve == BR_EC_SECP521R1:
+      c0.write(Asn1Tag.Oid, Asn1OidSecp521r1)
+    c0.finish()
+    offset = pubkey.getOffset()
+    length = pubkey.key.qlen
+    c1.write(Asn1Tag.BitString,
+             pubkey.buffer.toOpenArray(offset, offset + length - 1))
+    c1.finish()
+    offset = seckey.getOffset()
+    length = seckey.key.xlen
+    p.write(1'u64)
+    p.write(Asn1Tag.OctetString,
+            seckey.buffer.toOpenArray(offset, offset + length - 1))
+    p.write(c0)
+    p.write(c1)
+    p.finish()
+    b.write(p)
+    b.finish()
+    result = len(b)
+    if len(data) >= len(b):
+      copyMem(addr data[0], addr b.buffer[0], len(b))
 
-proc toBytes*(pubkey: EcPublicKey, data: var openarray[byte]): bool =
+proc toBytes*(pubkey: EcPublicKey, data: var openarray[byte]): int =
-  ## Serialize EC public key ``pubkey`` to raw binary form and store it to
+  ## Serialize EC public key ``pubkey`` to ASN.1 DER binary form and store it
-  ## ``data``.
+  ## to ``data``.
   ##
-  ## If ``pubkey`` curve is ``Secp256r1`` length of ``data`` array must be at
+  ## Procedure returns number of bytes (octets) needed to store EC public key,
-  ## least ``PubKey256Length``.
+  ## or `0` if public key is not in supported curve.
-  ##
-  ## If ``pubkey`` curve is ``Secp384r1`` length of ``data`` array must be at
-  ## least ``PubKey384Length``.
-  ##
-  ## If ``pubkey`` curve is ``Secp521r1`` length of ``data`` array must be at
-  ## least ``PubKey521Length``.
-  ##
-  ## Procedure returns ``true`` if serialization successfull, ``false``
-  ## otherwise.
   if pubkey.key.curve in EcSupportedCurvesCint:
-    if copyKey(data, pubkey):
+    var b = Asn1Buffer.init()
-      result = true
+    var p = Asn1Composite.init(Asn1Tag.Sequence)
+    var c = Asn1Composite.init(Asn1Tag.Sequence)
+    c.write(Asn1Tag.Oid, Asn1OidEcPublicKey)
+    if pubkey.key.curve == BR_EC_SECP256R1:
+      c.write(Asn1Tag.Oid, Asn1OidSecp256r1)
+    elif pubkey.key.curve == BR_EC_SECP384R1:
+      c.write(Asn1Tag.Oid, Asn1OidSecp384r1)
+    elif pubkey.key.curve == BR_EC_SECP521R1:
+      c.write(Asn1Tag.Oid, Asn1OidSecp521r1)
+    c.finish()
+    p.write(c)
+    let offset = getOffset(pubkey)
+    let length = pubkey.key.qlen
+    p.write(Asn1Tag.BitString,
+            pubkey.buffer.toOpenArray(offset, offset + length - 1))
+    p.finish()
+    b.write(p)
+    b.finish()
+    result = len(b)
+    if len(data) >= len(b):
+      copyMem(addr data[0], addr b.buffer[0], len(b))
 
 proc getBytes*(seckey: EcPrivateKey): seq[byte] =
-  ## Serialize EC private key ``seckey`` to raw binary form and return it.
+  ## Serialize EC private key ``seckey`` to ASN.1 DER binary form and return it.
   if seckey.key.curve in EcSupportedCurvesCint:
-    result = newSeq[byte](seckey.key.xlen)
+    result = newSeq[byte]()
-    discard toBytes(seckey, result)
+    let length = seckey.toBytes(result)
+    result.setLen(length)
+    discard seckey.toBytes(result)
   else:
     raise newException(EcKeyIncorrectError, "Incorrect private key")
 
 proc getBytes*(pubkey: EcPublicKey): seq[byte] =
-  ## Serialize EC public key ``pubkey`` to raw binary form and return it.
+  ## Serialize EC public key ``pubkey`` to ASN.1 DER binary form and return it.
   if pubkey.key.curve in EcSupportedCurvesCint:
-    result = newSeq[byte](pubkey.key.qlen)
+    result = newSeq[byte]()
-    discard toBytes(pubkey, result)
+    let length = pubkey.toBytes(result)
+    result.setLen(length)
+    discard pubkey.toBytes(result)
   else:
     raise newException(EcKeyIncorrectError, "Incorrect public key")
 
@@ -399,67 +417,127 @@ proc `==`*(sig1, sig2: EcSignature): bool =
     return false
   result = (sig1.buffer == sig2.buffer)
 
-proc init*(key: var EcPrivateKey, data: openarray[byte]): bool =
+proc init*(key: var EcPrivateKey, data: openarray[byte]): Asn1Status =
-  ## Initialize EC `private key` or `scalar` ``key`` from raw binary
+  ## Initialize EC `private key` or `scalar` ``key`` from ASN.1 DER binary
   ## representation ``data``.
   ##
-  ## Length of ``data`` array must be ``SecKey256Length``, ``SecKey384Length``
+  ## Procedure returns ``Asn1Status``.
-  ## or ``SecKey521Length``.
+  var raw, oid, field: Asn1Field
-  ##
-  ## Procedure returns ``true`` on success, ``false`` otherwise.
   var curve: cint
-  if len(data) == SecKey256Length:
-    curve = cast[cint](Secp256r1)
-    result = true
-  elif len(data) == SecKey384Length:
-    curve = cast[cint](Secp384r1)
-    result = true
-  elif len(data) == SecKey521Length:
-    curve = cast[cint](Secp521r1)
-    result = true
-  if result:
-    result = false
-    if checkScalar(data, curve) == 1'u32:
-      let length = len(data)
-      key = new EcPrivateKey
-      key.buffer = newSeq[byte](length)
-      copyMem(addr key.buffer[0], unsafeAddr data[0], length)
-      key.key.x = cast[ptr cuchar](addr key.buffer[0])
-      key.key.xlen = length
-      key.key.curve = curve
-      result = true
 
-proc init*(pubkey: var EcPublicKey, data: openarray[byte]): bool =
+  var ab = Asn1Buffer.init(data)
-  ## Initialize EC public key ``pubkey`` from raw binary representation
+
+  result = ab.read(field)
+  if result != Asn1Status.Success:
+    return
+  if field.kind != Asn1Tag.Sequence:
+    return Asn1Status.Incorrect
+
+  var ib = field.getBuffer()
+
+  result = ib.read(field)
+  if result != Asn1Status.Success:
+    return
+  if field.kind != Asn1Tag.Integer:
+    return Asn1Status.Incorrect
+  if field.vint != 1'u64:
+    return Asn1Status.Incorrect
+
+  result = ib.read(raw)
+  if result != Asn1Status.Success:
+    return
+  if raw.kind != Asn1Tag.OctetString:
+    return Asn1Status.Incorrect
+
+  result = ib.read(oid)
+  if result != Asn1Status.Success:
+    return
+  if oid.kind != Asn1Tag.Oid:
+    return Asn1Status.Incorrect
+
+  if oid == Asn1OidSecp256r1:
+    curve = cast[cint](Secp256r1)
+  elif oid == Asn1OidSecp384r1:
+    curve = cast[cint](Secp384r1)
+  elif oid == Asn1OidSecp521r1:
+    curve = cast[cint](Secp521r1)
+  else:
+    return Asn1Status.Incorrect
+
+  if checkScalar(raw.toOpenArray(), curve) == 1'u32:
+    key = new EcPrivateKey
+    key.buffer = newSeq[byte](raw.length)
+    copyMem(addr key.buffer[0], addr raw.buffer[raw.offset], raw.length)
+    key.key.x = cast[ptr cuchar](addr key.buffer[0])
+    key.key.xlen = raw.length
+    key.key.curve = curve
+    result = Asn1Status.Success
+  else:
+    result = Asn1Status.Incorrect
+
+proc init*(pubkey: var EcPublicKey, data: openarray[byte]): Asn1Status =
+  ## Initialize EC public key ``pubkey`` from ASN.1 DER binary representation
   ## ``data``.
   ##
-  ## Length of ``data`` array must be ``PubKey256Length``, ``PubKey384Length``
+  ## Procedure returns ``Asn1Status``.
-  ## or ``PubKey521Length``.
+  var raw, oid, field: Asn1Field
-  ##
-  ## Procedure returns ``true`` on success, ``false`` otherwise.
   var curve: cint
-  if len(data) > 0:
+
-    if data[0] == 0x04'u8:
+  var ab = Asn1Buffer.init(data)
-      if len(data) == PubKey256Length:
+  result = ab.read(field)
-        curve = cast[cint](Secp256r1)
+  if result != Asn1Status.Success:
-        result = true
+    return
-      elif len(data) == PubKey384Length:
+  if field.kind != Asn1Tag.Sequence:
-        curve = cast[cint](Secp384r1)
+    return Asn1Status.Incorrect
-        result = true
+
-      elif len(data) == PubKey521Length:
+  var ib = field.getBuffer()
-        curve = cast[cint](Secp521r1)
+  result = ib.read(field)
-        result = true
+  if result != Asn1Status.Success:
-  if result:
+    return
-    result = false
+  if field.kind != Asn1Tag.Sequence:
-    if checkPublic(data, curve) != 0:
+    return Asn1Status.Incorrect
-      let length = len(data)
+
-      pubkey = new EcPublicKey
+  var ob = field.getBuffer()
-      pubkey.buffer = newSeq[byte](length)
+  result = ob.read(oid)
-      copyMem(addr pubkey.buffer[0], unsafeAddr data[0], length)
+  if result != Asn1Status.Success:
-      pubkey.key.q = cast[ptr cuchar](addr pubkey.buffer[0])
+    return
-      pubkey.key.qlen = length
+  if oid.kind != Asn1Tag.Oid:
-      pubkey.key.curve = curve
+    return Asn1Status.Incorrect
-      result = true
+
+  if oid != Asn1OidEcPublicKey:
+    return Asn1Status.Incorrect
+
+  result = ob.read(oid)
+  if result != Asn1Status.Success:
+    return
+  if oid.kind != Asn1Tag.Oid:
+    return Asn1Status.Incorrect
+
+  if oid == Asn1OidSecp256r1:
+    curve = cast[cint](Secp256r1)
+  elif oid == Asn1OidSecp384r1:
+    curve = cast[cint](Secp384r1)
+  elif oid == Asn1OidSecp521r1:
+    curve = cast[cint](Secp521r1)
+  else:
+    return Asn1Status.Incorrect
+
+  result = ib.read(raw)
+  if result != Asn1Status.Success:
+    return
+  if raw.kind != Asn1Tag.BitString:
+    return Asn1Status.Incorrect
+
+  if checkPublic(raw.toOpenArray(), curve) != 0:
+    pubkey = new EcPublicKey
+    pubkey.buffer = newSeq[byte](raw.length)
+    copyMem(addr pubkey.buffer[0], addr raw.buffer[raw.offset], raw.length)
+    pubkey.key.q = cast[ptr cuchar](addr pubkey.buffer[0])
+    pubkey.key.qlen = raw.length
+    pubkey.key.curve = curve
+    result = Asn1Status.Success
+  else:
+    result = Asn1Status.Incorrect
 
 proc init*(sig: var EcSignature, data: openarray[byte]): bool =
   ## Initialize EC signature ``sig`` from raw binary representation ``data``.
@@ -493,16 +571,20 @@ proc init*[T: EcPKI](sospk: var T, data: string): bool {.inline.} =
   result = sospk.init(fromHex(data))
 
 proc init*(t: typedesc[EcPrivateKey], data: openarray[byte]): EcPrivateKey =
-  ## Initialize EC private key from raw binary representation ``data`` and
+  ## Initialize EC private key from ASN.1 DER binary representation ``data`` and
   ## return constructed object.
-  if not result.init(data):
+  let res = result.init(data)
-    raise newException(EcKeyIncorrectError, "Incorrect private key")
+  if res != Asn1Status.Success:
+    raise newException(EcKeyIncorrectError,
+                       "Incorrect private key (" & $res & ")")
 
 proc init*(t: typedesc[EcPublicKey], data: openarray[byte]): EcPublicKey =
-  ## Initialize EC public key from raw binary representation ``data`` and
+  ## Initialize EC public key from ASN.1 DER binary representation ``data`` and
   ## return constructed object.
-  if not result.init(data):
+  let res = result.init(data)
-    raise newException(EcKeyIncorrectError, "Incorrect public key")
+  if res != Asn1Status.Success:
+    raise newException(EcKeyIncorrectError,
+                       "Incorrect public key (" & $res & ")")
 
 proc init*(t: typedesc[EcSignature], data: openarray[byte]): EcSignature =
   ## Initialize EC signature from raw binary representation ``data`` and
@@ -585,3 +667,20 @@ proc verify*[T: byte|char](sig: EcSignature, message: openarray[T],
       # Clear context with initial value
       kv.init(addr hc.vtable)
       result = (res == 1)
+
+when isMainModule:
+  var buffer = newSeq[byte]()
+  var kp = EcKeyPair.random(Secp256r1)
+  var length: int
+
+  var serializedSK = kp.seckey.getBytes()
+  var serializedPK = kp.pubkey.getBytes()
+  echo toHex(serializedPK)
+  echo toHex(serializedSK)
+
+  var kp2 = EcPrivateKey.init(serializedSK)
+  echo toHex(kp2.getBytes())
+
+  var pk2 = EcPublicKey.init(serializedPK)
+  echo repr pk2
+  echo toHex(pk2.getBytes())

libp2p/crypto/minasn1.nim

@@ -0,0 +1,773 @@
+## Nim-Libp2p
+## Copyright (c) 2018 Status Research & Development GmbH
+## Licensed under either of
+##  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE))
+##  * MIT license ([LICENSE-MIT](LICENSE-MIT))
+## at your option.
+## This file may not be copied, modified, or distributed except according to
+## those terms.
+
+## This module implements minimal ASN.1 encoding/decoding primitives.
+import endians
+import nimcrypto/utils
+
+type
+  Asn1Status* {.pure.} = enum
+    Error,
+    Success,
+    Overflow,
+    Incomplete,
+    Indefinite,
+    Incorrect,
+    NoSupport,
+    Overrun
+
+  Asn1Class* {.pure.} = enum
+    Universal = 0x00,
+    Application = 0x01
+    ContextSpecific = 0x02
+    Private = 0x03
+
+  Asn1Tag* {.pure.} = enum
+    ## Protobuf's field types enum
+    NoSupport,
+    Boolean,
+    Integer,
+    BitString,
+    OctetString,
+    Null,
+    Oid,
+    Sequence,
+    Context
+
+  Asn1Buffer* = object of RootObj
+    ## ASN.1's message representation object
+    buffer*: seq[byte]
+    offset*: int
+    length*: int
+
+  Asn1Field* = object
+    klass*: Asn1Class
+    index*: int
+    offset*: int
+    length*: int
+    buffer*: seq[byte]
+    case kind*: Asn1Tag
+    of Asn1Tag.Boolean:
+      vbool*: bool
+    of Asn1Tag.Integer:
+      vint*: uint64
+    of Asn1Tag.BitString:
+      ubits*: int
+    else:
+      discard
+
+  Asn1Composite* = object of Asn1Buffer
+    tag*: Asn1Tag
+    idx*: int
+
+const
+  Asn1OidSecp256r1* = [
+    0x2A'u8, 0x86'u8, 0x48'u8, 0xCE'u8, 0x3D'u8, 0x03'u8, 0x01'u8, 0x07'u8
+  ]
+    ## Encoded OID for `secp256r1` curve (1.2.840.10045.3.1.7)
+  Asn1OidSecp384r1* = [
+    0x2B'u8, 0x81'u8, 0x04'u8, 0x00'u8, 0x22'u8
+  ]
+    ## Encoded OID for `secp384r1` curve (1.3.132.0.34)
+  Asn1OidSecp521r1* = [
+    0x2B'u8, 0x81'u8, 0x04'u8, 0x00'u8, 0x23'u8
+  ]
+    ## Encoded OID for `secp521r1` curve (1.3.132.0.35)
+  Asn1OidSecp256k1* = [
+    0x2B'u8, 0x81'u8, 0x04'u8, 0x00'u8, 0x0A'u8
+  ]
+    ## Encoded OID for `secp256k1` curve (1.3.132.0.10)
+  Asn1OidEcPublicKey* = [
+    0x2A'u8, 0x86'u8, 0x48'u8, 0xCE'u8, 0x3D'u8, 0x02'u8, 0x01'u8
+  ]
+    ## Encoded OID for Elliptic Curve Public Key (1.2.840.10045.2.1)
+  Asn1OidRsaEncryption* = [
+    0x2A'u8, 0x86'u8, 0x48'u8, 0x86'u8, 0xF7'u8, 0x0D'u8, 0x01'u8,
+    0x01'u8, 0x01'u8
+  ]
+    ## Encoded OID for RSA Encryption (1.2.840.113549.1.1.1)
+  Asn1True* = [0x01'u8, 0x01'u8, 0xFF'u8]
+    ## Encoded boolean ``TRUE``.
+  Asn1False* = [0x01'u8, 0x01'u8, 0x00'u8]
+    ## Encoded boolean ``FALSE``.
+  Asn1Null* = [0x05'u8, 0x00'u8]
+    ## Encoded ``NULL`` value.
+
+template toOpenArray*(ab: Asn1Buffer): untyped =
+  toOpenArray(ab.buffer, ab.offset, len(ab.buffer) - 1)
+
+template toOpenArray*(ac: Asn1Composite): untyped =
+  toOpenArray(ac.buffer, ac.offset, len(ac.buffer) - 1)
+
+template toOpenArray*(af: Asn1Field): untyped =
+  toOpenArray(af.buffer, af.offset, af.offset + af.length - 1)
+
+template isEmpty*(ab: Asn1Buffer): bool =
+  ab.offset >= len(ab.buffer)
+
+template isEnough*(ab: Asn1Buffer, length: int): bool =
+  len(ab.buffer) >= ab.offset + length
+
+proc len*[T: Asn1Buffer|Asn1Composite](abc: T): int {.inline.} =
+  len(abc.buffer) - abc.offset
+
+proc extend*[T: Asn1Buffer|Asn1Composite](abc: var T, length: int) {.inline.} =
+  ## Extend buffer or composite's internal buffer by ``length`` octets.
+  abc.buffer.setLen(len(abc.buffer) + length)
+
+proc code*(tag: Asn1Tag): byte {.inline.} =
+  ## Converts Nim ``tag`` enum to ASN.1 tag code.
+  case tag:
+  of Asn1Tag.NoSupport:
+    0x00'u8
+  of Asn1Tag.Boolean:
+    0x01'u8
+  of Asn1Tag.Integer:
+    0x02'u8
+  of Asn1Tag.BitString:
+    0x03'u8
+  of Asn1Tag.OctetString:
+    0x04'u8
+  of Asn1Tag.Null:
+    0x05'u8
+  of Asn1Tag.Oid:
+    0x06'u8
+  of Asn1Tag.Sequence:
+    0x30'u8
+  of Asn1Tag.Context:
+    0xA0'u8
+
+proc asn1EncodeLength*(dest: var openarray[byte], length: int64): int =
+  ## Encode ASN.1 DER length part of TLV triple and return number of bytes
+  ## (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``length`` value, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  if length < 0x80:
+    if len(dest) >= 1:
+      dest[0] = cast[byte](length)
+      result = 1
+  else:
+    result = 0
+    var z = length
+    while z != 0:
+      inc(result)
+      z = z shr 8
+    if len(dest) >= result + 1:
+      dest[0] = cast[byte](0x80 + result)
+      var o = 1
+      for j in countdown(result - 1, 0):
+        dest[o] = cast[byte](length shr (j shl 3))
+        inc(o)
+    inc(result)
+
+proc asn1EncodeInteger*(dest: var openarray[byte],
+                        value: openarray[byte]): int =
+  ## Encode big-endian binary representation of integer as ASN.1 DER `INTEGER`
+  ## and return number of bytes (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  var buffer: array[16, byte]
+  var o = 0
+  var lenlen = 0
+  for i in 0..<len(value):
+    if value[o] != 0x00:
+      break
+    inc(o)
+  if len(value) > 0:
+    if o == len(value):
+      dec(o)
+    if value[o] >= 0x80'u8:
+      lenlen = asn1EncodeLength(buffer, len(value) - o + 1)
+      result = 1 + lenlen + 1 + (len(value) - o)
+    else:
+      lenlen = asn1EncodeLength(buffer, len(value) - o)
+      result = 1 + lenlen + (len(value) - o)
+  else:
+    result = 2
+  if len(dest) >= result:
+    var s = 1
+    dest[0] = Asn1Tag.Integer.code()
+    copyMem(addr dest[1], addr buffer[0], lenlen)
+    if value[o] >= 0x80'u8:
+      dest[1 + lenlen] = 0x00'u8
+      s = 2
+    if len(value) > 0:
+      copyMem(addr dest[s + lenlen], unsafeAddr value[o], len(value) - o)
+
+proc asn1EncodeInteger*[T: SomeUnsignedInt](dest: var openarray[byte],
+                                            value: T): int =
+  ## Encode Nim's unsigned integer as ASN.1 DER `INTEGER` and return number of
+  ## bytes (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  when T is uint64:
+    var buffer: array[8, byte]
+    bigEndian64(addr buffer[0], cast[pointer](unsafeAddr value))
+    result = asn1EncodeInteger(dest, buffer)
+  elif T is uint32:
+    var buffer: array[4, byte]
+    bigEndian32(addr buffer[0], cast[pointer](unsafeAddr value))
+    result = asn1EncodeInteger(dest, buffer)
+  elif T is uint16:
+    var buffer: array[2, byte]
+    bigEndian16(addr buffer[0], cast[pointer](unsafeAddr value))
+    result = asn1EncodeInteger(dest, buffer)
+  elif T is uint8:
+    var buffer: array[1, byte]
+    buffer[0] = value
+    result = asn1EncodeInteger(dest, buffer)
+
+proc asn1EncodeBoolean*(dest: var openarray[byte], value: bool): int =
+  ## Encode Nim's boolean as ASN.1 DER `BOOLEAN` and return number of bytes
+  ## (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  result = 3
+  if len(dest) >= result:
+    dest[0] = Asn1Tag.Boolean.code()
+    dest[1] = 0x01'u8
+    dest[2] = if value: 0xFF'u8 else: 0x00'u8
+
+proc asn1EncodeNull*(dest: var openarray[byte]): int =
+  ## Encode ASN.1 DER `NULL` and return number of bytes (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  result = 2
+  if len(dest) >= result:
+    dest[0] = Asn1Tag.Null.code()
+    dest[1] = 0x00'u8
+
+proc asn1EncodeOctetString*(dest: var openarray[byte],
+                          value: openarray[byte]): int =
+  ## Encode array of bytes as ASN.1 DER `OCTET STRING` and return number of
+  ## bytes (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  var buffer: array[16, byte]
+  var lenlen = asn1EncodeLength(buffer, len(value))
+  result = 1 + lenlen + len(value)
+  if len(dest) >= result:
+    dest[0] = Asn1Tag.OctetString.code()
+    copyMem(addr dest[1], addr buffer[0], lenlen)
+    if len(value) > 0:
+      copyMem(addr dest[1 + lenlen], unsafeAddr value[0], len(value))
+
+proc asn1EncodeBitString*(dest: var openarray[byte],
+                          value: openarray[byte], bits = 0): int =
+  ## Encode array of bytes as ASN.1 DER `BIT STRING` and return number of bytes
+  ## (octets) used.
+  ##
+  ## ``bits`` number of used bits in ``value``. If ``bits == 0``, all the bits
+  ## from ``value`` are used, if ``bits != 0`` only number of ``bits`` will be
+  ## used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  var buffer: array[16, byte]
+  var lenlen = asn1EncodeLength(buffer, len(value) + 1)
+  var lbits = 0
+  if bits != 0:
+    lbits = len(value) shl 3 - bits
+  result = 1 + lenlen + 1 + len(value)
+  if len(dest) >= result:
+    dest[0] = Asn1Tag.BitString.code()
+    copyMem(addr dest[1], addr buffer[0], lenlen)
+    dest[1 + lenlen] = cast[byte](lbits)
+    if len(value) > 0:
+      copyMem(addr dest[2 + lenlen], unsafeAddr value[0], len(value))
+
+proc asn1EncodeTag[T: SomeUnsignedInt](dest: var openarray[byte],
+                                       value: T): int =
+  var v = value
+  if value <= cast[T](0x7F):
+    if len(dest) >= 1:
+      dest[0] = cast[byte](value)
+    result = 1
+  else:
+    var s = 0
+    while v != 0:
+      v = v shr 7
+      s += 7
+      inc(result)
+    if len(dest) >= result:
+      var k = 0
+      while s != 0:
+        s -= 7
+        dest[k] = cast[byte](((value shr s) and cast[T](0x7F)) or cast[T](0x80))
+        inc(k)
+      dest[k - 1] = dest[k - 1] and 0x7F'u8
+
+proc asn1EncodeOid*(dest: var openarray[byte], value: openarray[int]): int =
+  ## Encode array of integers ``value`` as ASN.1 DER `OBJECT IDENTIFIER` and
+  ## return number of bytes (octets) used.
+  ##
+  ## OBJECT IDENTIFIER requirements for ``value`` elements:
+  ##   * len(value) >= 2
+  ##   * value[0] >= 1 and value[0] < 2
+  ##   * value[1] >= 1 and value[1] < 39
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  var buffer: array[16, byte]
+  result = 1
+  assert(len(value) >= 2)
+  assert(value[0] >= 1 and value[0] < 2)
+  assert(value[1] >= 1 and value[1] <= 39)
+  var oidlen = 1
+  for i in 2..<len(value):
+    oidlen += asn1EncodeTag(buffer, cast[uint64](value[i]))
+  result += asn1EncodeLength(buffer, oidlen)
+  result += oidlen
+  if len(dest) >= result:
+    let last = len(dest) - 1
+    var offset = 1
+    dest[0] = Asn1Tag.Oid.code()
+    offset += asn1EncodeLength(dest.toOpenArray(offset, last), oidlen)
+    dest[offset] = cast[byte](value[0] * 40 + value[1])
+    offset += 1
+    for i in 2..<len(value):
+      offset += asn1EncodeTag(dest.toOpenArray(offset, last),
+                              cast[uint64](value[i]))
+
+proc asn1EncodeOid*(dest: var openarray[byte], value: openarray[byte]): int =
+  ## Encode array of bytes ``value`` as ASN.1 DER `OBJECT IDENTIFIER` and return
+  ## number of bytes (octets) used.
+  ##
+  ## This procedure is useful to encode constant predefined identifiers such
+  ## as ``asn1OidSecp256r1``, ``asn1OidRsaEncryption``.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  var buffer: array[16, byte]
+  var lenlen = asn1EncodeLength(buffer, len(value))
+  result = 1 + lenlen + len(value)
+  if len(dest) >= result:
+    dest[0] = Asn1Tag.Oid.code()
+    copyMem(addr dest[1], addr buffer[0], lenlen)
+    copyMem(addr dest[1 + lenlen], unsafeAddr value[0], len(value))
+
+proc asn1EncodeSequence*(dest: var openarray[byte],
+                         value: openarray[byte]): int =
+  ## Encode ``value`` as ASN.1 DER `SEQUENCE` and return number of bytes
+  ## (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  var buffer: array[16, byte]
+  var lenlen = asn1EncodeLength(buffer, len(value))
+  result = 1 + lenlen + len(value)
+  if len(dest) >= result:
+    dest[0] = Asn1Tag.Sequence.code()
+    copyMem(addr dest[1], addr buffer[0], lenlen)
+    copyMem(addr dest[1 + lenlen], unsafeAddr value[0], len(value))
+
+proc asn1EncodeComposite*(dest: var openarray[byte],
+                          value: Asn1Composite): int =
+  ## Encode composite value and return number of bytes (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  var buffer: array[16, byte]
+  var lenlen = asn1EncodeLength(buffer, len(value.buffer))
+  result = 1 + lenlen + len(value.buffer)
+  if len(dest) >= result:
+    dest[0] = value.tag.code()
+    copyMem(addr dest[1], addr buffer[0], lenlen)
+    copyMem(addr dest[1 + lenlen], unsafeAddr value.buffer[0],
+            len(value.buffer))
+
+proc asn1EncodeContextTag*(dest: var openarray[byte], value: openarray[byte],
+                           tag: int): int =
+  ## Encode ASN.1 DER `CONTEXT SPECIFIC TAG` ``tag`` for value ``value`` and
+  ## return number of bytes (octets) used.
+  ##
+  ## If length of ``dest`` is less then number of required bytes to encode
+  ## ``value``, then result of encoding will not be stored in ``dest``
+  ## but number of bytes (octets) required will be returned.
+  var buffer: array[16, byte]
+  var lenlen = asn1EncodeLength(buffer, len(value))
+  result = 1 + lenlen + len(value)
+  if len(dest) >= result:
+    dest[0] = 0xA0'u8 or (cast[byte](tag) and 0x0F)
+    copyMem(addr dest[1], addr buffer[0], lenlen)
+    copyMem(addr dest[1 + lenlen], unsafeAddr value[0], len(value))
+
+proc getLength(ab: var Asn1Buffer, length: var uint64): Asn1Status =
+  ## Decode length part of ASN.1 TLV triplet.
+  result = Asn1Status.Incomplete
+  if not ab.isEmpty():
+    let b = ab.buffer[ab.offset]
+    if (b and 0x80'u8) == 0x00'u8:
+      length = cast[uint64](b)
+      ab.offset += 1
+      result = Asn1Status.Success
+      return
+    if b == 0x80'u8:
+      length = 0'u64
+      result = Asn1Status.Indefinite
+      return
+    if b == 0xFF'u8:
+      length = 0'u64
+      result = Asn1Status.Incorrect
+      return
+    let octets = cast[uint64](b and 0x7F'u8)
+    if octets > 8'u64:
+      length = 0'u64
+      result = Asn1Status.Overflow
+      return
+    length = 0'u64
+    if ab.isEnough(int(octets)):
+      for i in 0..<int(octets):
+        length = (length shl 8) or cast[uint64](ab.buffer[ab.offset + i + 1])
+      ab.offset = ab.offset + int(octets) + 1
+      result = Asn1Status.Success
+
+proc getTag(ab: var Asn1Buffer, tag: var int,
+            klass: var Asn1Class): Asn1Status =
+  ## Decode tag part of ASN.1 TLV triplet.
+  result = Asn1Status.Incomplete
+  if not ab.isEmpty():
+    let b = ab.buffer[ab.offset]
+    var c = int((b and 0xC0'u8) shr 6)
+    if c >= 0 and c < 4:
+      klass = cast[Asn1Class](c)
+    else:
+      return Asn1Status.Incorrect
+    tag = int(b and 0x3F)
+    ab.offset += 1
+    result = Asn1Status.Success
+
+proc read*(ab: var Asn1Buffer, field: var Asn1Field): Asn1Status =
+  ## Decode value part of ASN.1 TLV triplet.
+  var
+    tag, ttag, offset: int
+    length, tlength: uint64
+    klass: Asn1Class
+    res: Asn1Status
+    inclass: bool
+
+  inclass = false
+  while true:
+    offset = ab.offset
+    result = ab.getTag(tag, klass)
+    if result != Asn1Status.Success:
+      break
+
+    if klass == Asn1Class.ContextSpecific:
+      if inclass:
+        result = Asn1Status.Incorrect
+        break
+      inclass = true
+      ttag = tag
+      result = ab.getLength(tlength)
+      if result != Asn1Status.Success:
+        break
+
+    elif klass == Asn1Class.Universal:
+      result = ab.getLength(length)
+      if result != Asn1Status.Success:
+        break
+
+      if inclass:
+        if length >= tlength:
+          result = Asn1Status.Incorrect
+          break
+
+      if cast[byte](tag) == Asn1Tag.Boolean.code():
+        # BOOLEAN
+        if length != 1:
+          result = Asn1Status.Incorrect
+          break
+        if not ab.isEnough(cast[int](length)):
+          result = Asn1Status.Incomplete
+          break
+        let b = ab.buffer[ab.offset]
+        if b != 0xFF'u8 and b != 0x00'u8:
+          result = Asn1Status.Incorrect
+          break
+        field = Asn1Field(kind: Asn1Tag.Boolean, klass: klass,
+                          index: ttag, offset: cast[int](ab.offset),
+                          length: 1)
+        shallowCopy(field.buffer, ab.buffer)
+        field.vbool = (b == 0xFF'u8)
+        ab.offset += 1
+        result = Asn1Status.Success
+        break
+      elif cast[byte](tag) == Asn1Tag.Integer.code():
+        # INTEGER
+        if not ab.isEnough(cast[int](length)):
+          result = Asn1Status.Incomplete
+          break
+        field = Asn1Field(kind: Asn1Tag.Integer, klass: klass,
+                          index: ttag, offset: cast[int](ab.offset),
+                          length: cast[int](length))
+        shallowCopy(field.buffer, ab.buffer)
+        if length <= 8:
+          for i in 0..<int(length):
+            field.vint = (field.vint shl 8) or
+                         cast[uint64](ab.buffer[ab.offset + i])
+        ab.offset += cast[int](length)
+        result = Asn1Status.Success
+        break
+      elif cast[byte](tag) == Asn1Tag.BitString.code():
+        # BIT STRING
+        if not ab.isEnough(cast[int](length)):
+          result = Asn1Status.Incomplete
+          break
+        field = Asn1Field(kind: Asn1Tag.BitString, klass: klass,
+                          index: ttag, offset: cast[int](ab.offset + 1),
+                          length: cast[int](length - 1))
+        shallowCopy(field.buffer, ab.buffer)
+        field.ubits = cast[int](((length - 1) shl 3) - ab.buffer[ab.offset])
+        ab.offset += cast[int](length)
+        result = Asn1Status.Success
+        break
+      elif cast[byte](tag) == Asn1Tag.OctetString.code():
+        # OCT STRING
+        if not ab.isEnough(cast[int](length)):
+          result = Asn1Status.Incomplete
+          break
+        field = Asn1Field(kind: Asn1Tag.OctetString, klass: klass,
+                          index: ttag, offset: cast[int](ab.offset),
+                          length: cast[int](length))
+        shallowCopy(field.buffer, ab.buffer)
+        ab.offset += cast[int](length)
+        result = Asn1Status.Success
+        break
+      elif cast[byte](tag) == Asn1Tag.Null.code():
+        # NULL
+        if length != 0:
+          result = Asn1Status.Incorrect
+          break
+        field = Asn1Field(kind: Asn1Tag.Null, klass: klass,
+                          index: ttag, offset: cast[int](ab.offset),
+                          length: 0)
+        shallowCopy(field.buffer, ab.buffer)
+        ab.offset += cast[int](length)
+        result = Asn1Status.Success
+        break
+      elif cast[byte](tag) == Asn1Tag.Oid.code():
+        # OID
+        if not ab.isEnough(cast[int](length)):
+          result = Asn1Status.Incomplete
+          break
+        field = Asn1Field(kind: Asn1Tag.Oid, klass: klass,
+                          index: ttag, offset: cast[int](ab.offset),
+                          length: cast[int](length))
+        shallowCopy(field.buffer, ab.buffer)
+        ab.offset += cast[int](length)
+        result = Asn1Status.Success
+        break
+      elif cast[byte](tag) == Asn1Tag.Sequence.code():
+        # SEQUENCE
+        if not ab.isEnough(cast[int](length)):
+          result = Asn1Status.Incomplete
+          break
+        field = Asn1Field(kind: Asn1Tag.Sequence, klass: klass,
+                          index: ttag, offset: cast[int](ab.offset),
+                          length: cast[int](length))
+        shallowCopy(field.buffer, ab.buffer)
+        ab.offset += cast[int](length)
+        result = Asn1Status.Success
+        break
+      else:
+        result = Asn1Status.NoSupport
+        break
+      inclass = false
+      ttag = 0
+    else:
+      result = Asn1Status.NoSupport
+      break
+
+proc getBuffer*(field: Asn1Field): Asn1Buffer =
+  ## Return ``field`` as Asn1Buffer to enter composite types.
+  shallowCopy(result.buffer, field.buffer)
+  result.offset = field.offset
+  result.length = field.length
+
+proc `==`*(field: Asn1Field, data: openarray[byte]): bool =
+  ## Compares field ``field`` data with ``data`` and returns ``true`` if both
+  ## buffers are equal.
+  let length = len(field.buffer)
+  if length > 0:
+    if field.length == len(data):
+      result = equalMem(unsafeAddr field.buffer[field.offset],
+                        unsafeAddr data[0], field.length)
+
+proc init*(t: typedesc[Asn1Buffer], data: openarray[byte]): Asn1Buffer =
+  ## Initialize ``Asn1Buffer`` from array of bytes ``data``.
+  result.buffer = @data
+
+proc init*(t: typedesc[Asn1Buffer], data: string): Asn1Buffer =
+  ## Initialize ``Asn1Buffer`` from hexadecimal string ``data``.
+  result.buffer = fromHex(data)
+
+proc init*(t: typedesc[Asn1Buffer]): Asn1Buffer =
+  ## Initialize empty ``Asn1Buffer``.
+  result.buffer = newSeq[byte]()
+
+proc init*(t: typedesc[Asn1Composite], tag: Asn1Tag): Asn1Composite =
+  ## Initialize ``Asn1Composite`` with tag ``tag``.
+  result.tag = tag
+  result.buffer = newSeq[byte]()
+
+proc init*(t: typedesc[Asn1Composite], idx: int): Asn1Composite =
+  ## Initialize ``Asn1Composite`` with tag context-specific id ``id``.
+  result.tag = Asn1Tag.Context
+  result.idx = idx
+  result.buffer = newSeq[byte]()
+
+proc `$`*(buffer: Asn1Buffer): string =
+  ## Return string representation of ``buffer``.
+  result = toHex(buffer.toOpenArray())
+
+proc `$`*(field: Asn1Field): string =
+  ## Return string representation of ``field``.
+  result = "["
+  result.add($field.kind)
+  result.add("]")
+  if field.kind == Asn1Tag.NoSupport:
+    result.add(" ")
+    result.add(toHex(field.toOpenArray()))
+  elif field.kind == Asn1Tag.Boolean:
+    result.add(" ")
+    result.add($field.vbool)
+  elif field.kind == Asn1Tag.Integer:
+    result.add(" ")
+    if field.length <= 8:
+      result.add($field.vint)
+    else:
+      result.add(toHex(field.toOpenArray()))
+  elif field.kind == Asn1Tag.BitString:
+    result.add(" ")
+    result.add("(")
+    result.add($field.ubits)
+    result.add(" bits) ")
+    result.add(toHex(field.toOpenArray()))
+  elif field.kind == Asn1Tag.OctetString:
+    result.add(" ")
+    result.add(toHex(field.toOpenArray()))
+  elif field.kind == Asn1Tag.Null:
+    result.add(" NULL")
+  elif field.kind == Asn1Tag.Oid:
+    result.add(" ")
+    result.add(toHex(field.toOpenArray()))
+  elif field.kind == Asn1Tag.Sequence:
+    result.add(" ")
+    result.add(toHex(field.toOpenArray()))
+
+proc write*[T: Asn1Buffer|Asn1Composite](abc: var T, tag: Asn1Tag) =
+  ## Write empty value to buffer or composite with ``tag``.
+  ##
+  ## This procedure must be used to write `NULL`, `0` or empty `BIT STRING`,
+  ## `OCTET STRING` types.
+  assert(tag in {Asn1Tag.Null, Asn1Tag.Integer, Asn1Tag.BitString,
+                 Asn1Tag.OctetString})
+  var length: int
+  if tag == Asn1Tag.Null:
+    length = asn1EncodeNull(abc.toOpenArray())
+    abc.extend(length)
+    discard asn1EncodeNull(abc.toOpenArray())
+  elif tag == Asn1Tag.Integer:
+    length = asn1EncodeInteger(abc.toOpenArray(), 0'u64)
+    abc.extend(length)
+    discard asn1EncodeInteger(abc.toOpenArray(), 0'u64)
+  elif tag == Asn1Tag.BitString:
+    var tmp: array[1, byte]
+    length = asn1EncodeBitString(abc.toOpenArray(), tmp.toOpenArray(0, -1))
+    abc.extend(length)
+    discard asn1EncodeBitString(abc.toOpenArray(), tmp.toOpenArray(0, -1))
+  elif tag == Asn1Tag.OctetString:
+    var tmp: array[1, byte]
+    length = asn1EncodeOctetString(abc.toOpenArray(), tmp.toOpenArray(0, -1))
+    abc.extend(length)
+    discard asn1EncodeOctetString(abc.toOpenArray(), tmp.toOpenArray(0, -1))
+  abc.offset += length
+
+proc write*[T: Asn1Buffer|Asn1Composite](abc: var T, value: uint64) =
+  ## Write uint64 ``value`` to buffer or composite as ASN.1 `INTEGER`.
+  let length = asn1EncodeInteger(abc.toOpenArray(), value)
+  abc.extend(length)
+  discard asn1EncodeInteger(abc.toOpenArray(), value)
+  abc.offset += length
+
+proc write*[T: Asn1Buffer|Asn1Composite](abc: var T, value: bool) =
+  ## Write bool ``value`` to buffer or composite as ASN.1 `BOOLEAN`.
+  let length = asn1EncodeBoolean(abc.toOpenArray(), value)
+  abc.extend(length)
+  discard asn1EncodeBoolean(abc.toOpenArray(), value)
+  abc.offset += length
+
+proc write*[T: Asn1Buffer|Asn1Composite](abc: var T, tag: Asn1Tag,
+                                         value: openarray[byte], bits = 0) =
+  ## Write array ``value`` using ``tag``.
+  ##
+  ## This procedure is used to write ASN.1 `INTEGER`, `OCTET STRING`,
+  ## `BIT STRING` or `OBJECT IDENTIFIER`.
+  ##
+  ## For `BIT STRING` you can use ``bits`` argument to specify number of used
+  ## bits.
+  assert(tag in {Asn1Tag.Integer, Asn1Tag.OctetString, Asn1Tag.BitString,
+                 Asn1Tag.Oid})
+  var length: int
+  if tag == Asn1Tag.Integer:
+    length = asn1EncodeInteger(abc.toOpenArray(), value)
+    abc.extend(length)
+    discard asn1EncodeInteger(abc.toOpenArray(), value)
+  elif tag == Asn1Tag.OctetString:
+    length = asn1EncodeOctetString(abc.toOpenArray(), value)
+    abc.extend(length)
+    discard asn1EncodeOctetString(abc.toOpenArray(), value)
+  elif tag == Asn1Tag.BitString:
+    length = asn1EncodeBitString(abc.toOpenArray(), value, bits)
+    abc.extend(length)
+    discard asn1EncodeBitString(abc.toOpenArray(), value, bits)
+  elif tag == Asn1Tag.Oid:
+    length = asn1EncodeOid(abc.toOpenArray(), value)
+    abc.extend(length)
+    discard asn1EncodeOid(abc.toOpenArray(), value)
+  abc.offset += length
+
+proc write*[T: Asn1Buffer|Asn1Composite](abc: var T, value: Asn1Composite) =
+  assert(len(value) > 0, "Composite value not finished")
+  var length: int
+  if value.tag == Asn1Tag.Sequence:
+    length = asn1EncodeSequence(abc.toOpenArray(), value.buffer)
+    abc.extend(length)
+    discard asn1EncodeSequence(abc.toOpenArray(), value.buffer)
+  elif value.tag == Asn1Tag.BitString:
+    length = asn1EncodeBitString(abc.toOpenArray(), value.buffer)
+    abc.extend(length)
+    discard asn1EncodeBitString(abc.toOpenArray(), value.buffer)
+  elif value.tag == Asn1Tag.Context:
+    length = asn1EncodeContextTag(abc.toOpenArray(), value.buffer, value.idx)
+    abc.extend(length)
+    discard asn1EncodeContextTag(abc.toOpenArray(), value.buffer, value.idx)
+  abc.offset += length
+
+proc finish*[T: Asn1Buffer|Asn1Composite](abc: var T) {.inline.} =
+  ## Finishes buffer or composite and prepares it for writing.
+  abc.offset = 0