diff --git a/codex/encryption/bearsslhash.nim b/codex/encryption/bearsslhash.nim
new file mode 100644
index 00000000..83d1d003
--- /dev/null
+++ b/codex/encryption/bearsslhash.nim
@@ -0,0 +1,12 @@
+import bearssl/hash
+
+proc bearSslHash*(hashClass: ptr HashClass, data: openArray[byte]): seq[byte] =
+  var compatCtx = HashCompatContext()
+  let buffSize = (hashClass[].desc shr HASHDESC_OUT_OFF) and HASHDESC_OUT_MASK
+  result = newSeq[byte](buffSize)
+
+  let hashClassPtrPtr: ConstPtrPtrHashClass = addr(compatCtx.vtable)
+
+  hashClass[].init(hashClassPtrPtr)
+  hashClass[].update(hashClassPtrPtr, addr data[0], data.len.uint)
+  hashClass[].`out`(hashClassPtrPtr, addr result[0])
diff --git a/codex/encryption/codexencryption.nim b/codex/encryption/codexencryption.nim
new file mode 100644
index 00000000..27e58fbd
--- /dev/null
+++ b/codex/encryption/codexencryption.nim
@@ -0,0 +1,68 @@
+## Nim-Codex
+## Copyright (c) 2025 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.
+
+import std/sequtils
+import bearssl/[blockx, hash]
+import stew/[byteutils, endians2]
+
+import ../rng
+import ./bearsslhash
+
+{.push raises: [].}
+
+const
+  MasterKeySize = 32 # 256 bits
+  KeySize = 24 # 192 bits for AES-192
+  IvSize = 16 # 128 bits
+  KeyDerivationIdentifier = "aes192_block_key".toBytes
+  IvDerivationIdentifier = "aes192_block_iv".toBytes
+
+type CodexEncryption* = ref object
+  masterKey: seq[byte]
+
+proc newCodexEncryption*(): CodexEncryption =
+  let masterKey = newSeqWith(32, Rng.instance.rand(uint8.high).byte)
+  CodexEncryption(masterKey: masterKey)
+
+proc newCodexEncryption*(masterKey: seq[byte]): CodexEncryption =
+  CodexEncryption(masterKey: masterKey)
+
+proc deriveKeyForBlockIndex(self: CodexEncryption, blockIndex: uint32): seq[byte] =
+  let blockIndexArray = toBytes(blockIndex, bigEndian)
+  bearSslHash(
+    addr sha256Vtable, self.masterKey & KeyDerivationIdentifier & blockIndexArray.toSeq
+  )[0 ..< KeySize]
+
+proc deriveIvForBlockIndex(self: CodexEncryption, blockIndex: uint32): seq[byte] =
+  let blockIndexArray = toBytes(blockIndex, bigEndian)
+  bearSslHash(
+    addr sha256Vtable, self.masterKey & IvDerivationIdentifier & blockIndexArray.toSeq
+  )[0 ..< IvSize]
+
+proc encryptBlock*(
+    self: CodexEncryption, blockData: seq[byte], blockIndex: uint32
+): seq[byte] =
+  let key = self.deriveKeyForBlockIndex(blockIndex)
+  let iv = self.deriveIvForBlockIndex(blockIndex)
+  result = blockData
+
+  var encCtx: AesBigCbcencKeys
+  aesBigCbcencInit(encCtx, addr key[0], key.len.uint)
+  aesBigCbcencRun(encCtx, addr iv[0], addr result[0], iv.len.uint)
+
+proc decryptBlock*(
+    self: CodexEncryption, encryptedBlockData: seq[byte], blockIndex: uint32
+): seq[byte] =
+  let key = self.deriveKeyForBlockIndex(blockIndex)
+  let iv = self.deriveIvForBlockIndex(blockIndex)
+  result = encryptedBlockData
+
+  var decCtx: AesBigCbcdecKeys
+  aesBigCbcdecInit(decCtx, addr key[0], key.len.uint)
+  aesBigCbcdecRun(decCtx, addr iv[0], addr result[0], iv.len.uint)