diff --git a/core/account/src/account_log.rs b/core/account/src/account_log.rs
new file mode 100644
index 0000000..80d0ceb
--- /dev/null
+++ b/core/account/src/account_log.rs
@@ -0,0 +1,177 @@
+//! Signed account operation log: the append-only record of an account's
+//! associated key and data.
+//!
+//! ```text
+//! SignedAccountLog          payload + account signature over its exact bytes
+//! └── EncodedAccountLog     the log as canonical bytes (wire form — see codec)
+//!     └── AccountLog        the log as validated entries (working form)
+//!         └── AccountEntry      Add(EntryData) | Remove { index }
+//!             └── EntryData     Ed25519Key | Text
+//! ```
+//!
+//! Invariants:
+//! - Append-only: a newer log strictly extends the older one
+//!   ([`verify_extension`](crate::verify_extension)). There is no version
+//!   counter — a longer log is a newer log. A log that is longer but does not
+//!   extend the old one has rewritten history: either the signer is showing
+//!   different histories to different readers, or the account key is
+//!   compromised.
+//! - A `Remove` tombstones a strictly earlier, still-live `Add`; anything else
+//!   rejects the whole log — fail closed ([`AccountLog::new`]).
+//!
+//! Replaying ([`AccountLog::live_entries`]) yields the account's current state.
+
+use crypto::Ed25519Signature;
+
+use crate::error::AccountLogError;
+
+/// An [`AccountLog`] in its canonical byte encoding, plus the account's
+/// signature over exactly those bytes.
+///
+/// The account key is not carried: the account address *is* the verifying key,
+/// supplied by the caller on verify.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SignedAccountLog {
+    pub payload: EncodedAccountLog,
+    pub signature: Ed25519Signature,
+}
+
+/// An [`AccountLog`] as canonical bytes — exactly what is signed and
+/// transmitted. Holding one proves the bytes decode to a valid log: construct
+/// via [`AccountLog::encode`] or [`parse`](Self::parse). Byte layout: see
+/// [`AccountLog::encode`].
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct EncodedAccountLog(pub(crate) Vec<u8>);
+
+/// The log as a validated entry list. Construction checks every `Remove`, so
+/// a held log always replays ([`live_entries`](Self::live_entries) cannot
+/// fail).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct AccountLog {
+    entries: Vec<AccountEntry>,
+}
+
+/// One operation in the log. An entry's index is its position — derived, not
+/// stored, so an entry cannot lie about where it sits. One enum rather than
+/// separate op/data fields: illegal combinations are unrepresentable.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum AccountEntry {
+    /// Endorse new data under this account.
+    Add(EntryData),
+    /// Tombstone the `Add` at position `index`. Must point at a strictly
+    /// earlier, still-live `Add`; anything else rejects the whole log —
+    /// fail closed, so verifiers can never skip their way to different sets.
+    Remove { index: u32 },
+}
+
+/// Data an account can endorse.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum EntryData {
+    /// A device (LocalIdentity) verifying key.
+    Ed25519Key([u8; 32]),
+    /// An arbitrary UTF-8 record.
+    Text(String),
+}
+
+impl AccountLog {
+    /// Validate `entries` as a log (see [`AccountEntry::Remove`]).
+    /// [`EncodedAccountLog::parse`] applies the same gate to received bytes,
+    /// so a broken log — un-rewritable once published — never gets published.
+    pub fn new(entries: Vec<AccountEntry>) -> Result<Self, AccountLogError> {
+        removed_flags(&entries)?;
+        Ok(Self { entries })
+    }
+
+    pub fn entries(&self) -> &[AccountEntry] {
+        &self.entries
+    }
+
+    /// Replay the log into its live entry set — the account's current state,
+    /// in add order.
+    pub fn live_entries(&self) -> Vec<EntryData> {
+        let removed = removed_flags(&self.entries).expect("validated at construction");
+        self.entries
+            .iter()
+            .enumerate()
+            .filter_map(|(i, entry)| match entry {
+                AccountEntry::Add(data) if !removed[i] => Some(data.clone()),
+                _ => None,
+            })
+            .collect()
+    }
+}
+
+/// Which entries have been tombstoned, or the `Remove` that broke the log.
+/// `index < position` means every target was already seen: single-pass.
+fn removed_flags(entries: &[AccountEntry]) -> Result<Vec<bool>, AccountLogError> {
+    let mut removed = vec![false; entries.len()];
+    for (position, entry) in entries.iter().enumerate() {
+        let AccountEntry::Remove { index } = entry else {
+            continue;
+        };
+        let target = *index as usize;
+        let targets_live_add = target < position
+            && !removed[target]
+            && matches!(entries[target], AccountEntry::Add(_));
+        if !targets_live_add {
+            return Err(AccountLogError::InvalidRemove {
+                position,
+                index: *index,
+            });
+        }
+        removed[target] = true;
+    }
+    Ok(removed)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn key(byte: u8) -> AccountEntry {
+        AccountEntry::Add(EntryData::Ed25519Key([byte; 32]))
+    }
+
+    /// Replay applies tombstones and preserves add order.
+    #[test]
+    fn live_entries_applies_removes() {
+        let log = AccountLog::new(vec![
+            key(1),
+            AccountEntry::Add(EntryData::Text("name".into())),
+            AccountEntry::Remove { index: 0 },
+            key(2),
+        ])
+        .unwrap();
+        assert_eq!(
+            log.live_entries(),
+            vec![
+                EntryData::Text("name".into()),
+                EntryData::Ed25519Key([2; 32]),
+            ]
+        );
+    }
+
+    /// Every malformed remove rejects the whole log: forward and self
+    /// references, removing a remove, and removing twice.
+    #[test]
+    fn new_rejects_invalid_removes() {
+        let dangling = vec![key(1), AccountEntry::Remove { index: 7 }];
+        let self_ref = vec![AccountEntry::Remove { index: 0 }];
+        let of_remove = vec![
+            key(1),
+            AccountEntry::Remove { index: 0 },
+            AccountEntry::Remove { index: 1 },
+        ];
+        let twice = vec![
+            key(1),
+            AccountEntry::Remove { index: 0 },
+            AccountEntry::Remove { index: 0 },
+        ];
+        for entries in [dangling, self_ref, of_remove, twice] {
+            assert!(matches!(
+                AccountLog::new(entries),
+                Err(AccountLogError::InvalidRemove { .. })
+            ));
+        }
+    }
+}
diff --git a/core/account/src/addr.rs b/core/account/src/addr.rs
new file mode 100644
index 0000000..2c0c9bc
--- /dev/null
+++ b/core/account/src/addr.rs
@@ -0,0 +1,68 @@
+use std::fmt;
+
+use crypto::Ed25519VerifyingKey;
+
+use crate::error::AccountError;
+
+/// A routable representation of an account
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct AccountAddr {
+    pubkey: Ed25519VerifyingKey,
+}
+
+impl AccountAddr {
+    pub fn to_bytes(&self) -> &[u8] {
+        self.pubkey.as_ref()
+    }
+}
+
+/// Displays as the hex of the account verifying key — the same form the
+/// directory and keypackage registry use for ids.
+impl fmt::Display for AccountAddr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&hex::encode(self.pubkey.as_ref()))
+    }
+}
+
+impl From<&Ed25519VerifyingKey> for AccountAddr {
+    fn from(value: &Ed25519VerifyingKey) -> Self {
+        Self {
+            pubkey: value.clone(),
+        }
+    }
+}
+
+/// Not every byte string is an address: exactly 32 bytes forming a valid
+/// Ed25519 key.
+impl TryFrom<&[u8]> for AccountAddr {
+    type Error = AccountError;
+
+    fn try_from(value: &[u8]) -> Result<Self, Self::Error> {
+        let bytes: [u8; 32] = value
+            .try_into()
+            .map_err(|_| AccountError::InvalidAddress)?;
+        let pubkey =
+            Ed25519VerifyingKey::from_bytes(&bytes).map_err(|_| AccountError::InvalidAddress)?;
+        Ok(Self { pubkey })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crypto::Ed25519SigningKey;
+
+    /// Display is the hex of the key; TryFrom round-trips the bytes.
+    #[test]
+    fn display_and_try_from_roundtrip() {
+        let key = Ed25519SigningKey::generate().verifying_key();
+        let addr = AccountAddr::from(&key);
+        assert_eq!(addr.to_string(), hex::encode(key.as_ref()));
+        assert_eq!(AccountAddr::try_from(addr.to_bytes()).unwrap(), addr);
+    }
+
+    #[test]
+    fn try_from_rejects_wrong_length() {
+        assert!(AccountAddr::try_from(&[0u8; 31][..]).is_err());
+    }
+}
diff --git a/core/account/src/codec.rs b/core/account/src/codec.rs
new file mode 100644
index 0000000..919cd65
--- /dev/null
+++ b/core/account/src/codec.rs
@@ -0,0 +1,409 @@
+//! Wire format for [`EncodedAccountLog`]: the canonical byte encoding of an
+//! [`AccountLog`]. Encoding and decoding live together so they cannot drift
+//! apart. The bytes are opaque to the server except for the fixed-offset
+//! header the extension check reads.
+
+use crypto::Ed25519VerifyingKey;
+
+use crate::error::AccountLogError;
+use crate::account_log::{AccountEntry, AccountLog, EncodedAccountLog, EntryData, SignedAccountLog};
+
+/// Domain-separation tag, prepended to every signed payload:
+///
+/// ```text
+/// logos:accounts:<version>\0
+/// ```
+///
+/// Binds the signature to this exact purpose; bump `<version>` on layout change.
+pub const ACCOUNT_LOG_DOMAIN: &[u8] = b"logos:accounts:1\0";
+
+/// [`ACCOUNT_LOG_DOMAIN`] without the version segment.
+const DOMAIN_STEM: &[u8] = b"logos:accounts:";
+
+// Entry wire tags. One tag byte determines exactly how many bytes follow, so
+// every byte string parses one way.
+const TAG_ADD: u8 = 1;
+const TAG_REMOVE: u8 = 2;
+const DATA_ED25519: u8 = 1;
+const DATA_TEXT: u8 = 2;
+
+/// Header bytes after the domain prefix: the entry count (u32 LE). The count
+/// doubles as the freshness marker; u32 so no plausible client bug can
+/// exhaust it (a u16 could be burned by a publish loop, and the counter has
+/// no reset mechanism).
+const HEADER: usize = 4;
+
+impl AccountLog {
+    /// Canonical binary encoding — the bytes that are both signed and
+    /// transmitted:
+    ///
+    /// ```text
+    /// domain  : ACCOUNT_LOG_DOMAIN            (constant prefix incl. version, NUL-terminated)
+    /// count   : u32 LE    (4 bytes)   — number of entries that follow
+    /// entries : count entries, each:
+    ///   0x01 0x01 <32 bytes>              Add(Ed25519Key)
+    ///   0x01 0x02 <u16 LE len> <bytes>    Add(Text), UTF-8
+    ///   0x02 <u32 LE index>               Remove
+    /// ```
+    ///
+    /// The account key is *not* embedded: the account is identified
+    /// out-of-band by the account verifying key the caller requests, and
+    /// [`verify_log`] checks the signature under that key — so a log for one
+    /// account cannot be passed off as another's.
+    pub fn encode(&self) -> EncodedAccountLog {
+        let entries = self.entries();
+        let mut out = Vec::with_capacity(ACCOUNT_LOG_DOMAIN.len() + HEADER + entries.len() * 34);
+        out.extend_from_slice(ACCOUNT_LOG_DOMAIN);
+        out.extend_from_slice(&(entries.len() as u32).to_le_bytes());
+        for entry in entries {
+            match entry {
+                AccountEntry::Add(EntryData::Ed25519Key(key)) => {
+                    out.push(TAG_ADD);
+                    out.push(DATA_ED25519);
+                    out.extend_from_slice(key);
+                }
+                AccountEntry::Add(EntryData::Text(text)) => {
+                    out.push(TAG_ADD);
+                    out.push(DATA_TEXT);
+                    out.extend_from_slice(&(text.len() as u16).to_le_bytes());
+                    out.extend_from_slice(text.as_bytes());
+                }
+                AccountEntry::Remove { index } => {
+                    out.push(TAG_REMOVE);
+                    out.extend_from_slice(&index.to_le_bytes());
+                }
+            }
+        }
+        EncodedAccountLog(out)
+    }
+}
+
+impl EncodedAccountLog {
+    /// Validate received bytes: checks the domain prefix and version, parses
+    /// exactly the declared number of entries with no bytes left over, and
+    /// validates the log itself via [`AccountLog::new`].
+    pub fn parse(bytes: Vec<u8>) -> Result<Self, AccountLogError> {
+        AccountLog::new(decode_entries(&bytes)?)?;
+        Ok(Self(bytes))
+    }
+
+    /// Decode. Cannot fail: construction validated the bytes.
+    pub fn decode(&self) -> AccountLog {
+        AccountLog::new(decode_entries(&self.0).expect("validated at construction"))
+            .expect("validated at construction")
+    }
+
+    /// The exact bytes that are signed and transmitted.
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.0
+    }
+
+    /// The entry count from the header — the freshness marker.
+    fn count(&self) -> u32 {
+        let at = ACCOUNT_LOG_DOMAIN.len();
+        u32::from_le_bytes(self.0[at..at + 4].try_into().expect("4 bytes"))
+    }
+
+    /// The entry bytes after the header — the region the extension check
+    /// compares.
+    fn entry_bytes(&self) -> &[u8] {
+        &self.0[ACCOUNT_LOG_DOMAIN.len() + HEADER..]
+    }
+}
+
+/// Decoder behind [`EncodedAccountLog::parse`] and [`EncodedAccountLog::decode`].
+fn decode_entries(payload: &[u8]) -> Result<Vec<AccountEntry>, AccountLogError> {
+    let payload = match payload.strip_prefix(ACCOUNT_LOG_DOMAIN) {
+        Some(rest) => rest,
+        None => return Err(domain_error(payload)),
+    };
+    if payload.len() < HEADER {
+        return Err(AccountLogError::Short);
+    }
+    let count = u32::from_le_bytes(payload[..4].try_into().expect("4 bytes")) as usize;
+
+    let mut body = &payload[HEADER..];
+    let mut entries = Vec::with_capacity(count.min(1024));
+    for _ in 0..count {
+        let (entry, rest) = decode_entry(body)?;
+        entries.push(entry);
+        body = rest;
+    }
+    if !body.is_empty() {
+        return Err(AccountLogError::Trailing);
+    }
+    Ok(entries)
+}
+
+/// Parse one entry off the front of `body`, returning it and the rest.
+fn decode_entry(body: &[u8]) -> Result<(AccountEntry, &[u8]), AccountLogError> {
+    let (&tag, body) = body.split_first().ok_or(AccountLogError::Short)?;
+    match tag {
+        TAG_ADD => {
+            let (&data_tag, body) = body.split_first().ok_or(AccountLogError::Short)?;
+            match data_tag {
+                DATA_ED25519 => {
+                    let (key, rest) = split_at_checked(body, 32)?;
+                    let key = key.try_into().expect("split yields 32 bytes");
+                    Ok((AccountEntry::Add(EntryData::Ed25519Key(key)), rest))
+                }
+                DATA_TEXT => {
+                    let (len, body) = split_at_checked(body, 2)?;
+                    let len = u16::from_le_bytes(len.try_into().expect("2 bytes")) as usize;
+                    let (text, rest) = split_at_checked(body, len)?;
+                    let text = String::from_utf8(text.to_vec()).map_err(|_| AccountLogError::Utf8)?;
+                    Ok((AccountEntry::Add(EntryData::Text(text)), rest))
+                }
+                other => Err(AccountLogError::Tag(other)),
+            }
+        }
+        TAG_REMOVE => {
+            let (index, rest) = split_at_checked(body, 4)?;
+            let index = u32::from_le_bytes(index.try_into().expect("4 bytes"));
+            Ok((AccountEntry::Remove { index }, rest))
+        }
+        other => Err(AccountLogError::Tag(other)),
+    }
+}
+
+/// Classify a payload that failed the domain check: our stem with a different
+/// version segment, or a foreign domain altogether.
+fn domain_error(payload: &[u8]) -> AccountLogError {
+    let Some(rest) = payload.strip_prefix(DOMAIN_STEM) else {
+        return AccountLogError::Domain;
+    };
+    match rest.iter().take(16).position(|&b| b == 0) {
+        Some(end) => AccountLogError::Version(String::from_utf8_lossy(&rest[..end]).into_owned()),
+        None => AccountLogError::Domain,
+    }
+}
+
+/// `split_at` that reports a truncated payload instead of panicking.
+fn split_at_checked(body: &[u8], mid: usize) -> Result<(&[u8], &[u8]), AccountLogError> {
+    if body.len() < mid {
+        return Err(AccountLogError::Short);
+    }
+    Ok(body.split_at(mid))
+}
+
+/// Verify the account signature over the exact payload bytes, returning the
+/// decoded log.
+///
+/// Verifying under the *requested* account key is what binds the log to that
+/// account: another account's validly-signed log won't verify under this key,
+/// so an untrusted server cannot substitute one.
+pub fn verify_log(
+    expected_account: &Ed25519VerifyingKey,
+    log: &SignedAccountLog,
+) -> Result<AccountLog, AccountLogError> {
+    expected_account
+        .verify(log.payload.as_bytes(), &log.signature)
+        .map_err(|_| AccountLogError::SignatureInvalid)?;
+    Ok(log.payload.decode())
+}
+
+/// Check that `new` strictly extends `old`: strictly more entries, and the old
+/// entry bytes are a prefix of the new ones. (The count field itself changes
+/// between versions, so the check is over the entry region, not the whole
+/// payload.)
+///
+/// The server runs this on publish to refuse stale or rewritten logs, and
+/// consumers run it against the last log they saw as defence in depth. It
+/// compares bytes, so the server needs no knowledge of entry semantics.
+pub fn verify_extension(old: &EncodedAccountLog, new: &EncodedAccountLog) -> Result<(), AccountLogError> {
+    if new.count() <= old.count() {
+        return Err(AccountLogError::NotLonger {
+            old: old.count(),
+            new: new.count(),
+        });
+    }
+    if !new.entry_bytes().starts_with(old.entry_bytes()) {
+        return Err(AccountLogError::Forked);
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crypto::Ed25519SigningKey;
+
+    fn key(byte: u8) -> AccountEntry {
+        AccountEntry::Add(EntryData::Ed25519Key([byte; 32]))
+    }
+
+    fn make_log(entries: Vec<AccountEntry>) -> AccountLog {
+        AccountLog::new(entries).unwrap()
+    }
+
+    /// encode → decode round-trips, and parse accepts encode's bytes,
+    /// including the empty log and every variant.
+    #[test]
+    fn payload_roundtrips() {
+        let log = make_log(vec![
+            key(1),
+            AccountEntry::Add(EntryData::Text("display name".into())),
+            AccountEntry::Remove { index: 0 },
+            key(2),
+        ]);
+        let payload = log.encode();
+        assert_eq!(payload.decode(), log);
+        assert_eq!(
+            EncodedAccountLog::parse(payload.as_bytes().to_vec()).unwrap(),
+            payload
+        );
+
+        // Empty log is valid (an account with no entries yet).
+        assert!(make_log(vec![]).encode().decode().entries().is_empty());
+    }
+
+    #[test]
+    fn parse_rejects_short_and_truncated() {
+        // A domain-prefixed payload too short to hold the header.
+        let mut short = ACCOUNT_LOG_DOMAIN.to_vec();
+        short.extend_from_slice(&[0u8; 3]);
+        assert!(matches!(
+            EncodedAccountLog::parse(short),
+            Err(AccountLogError::Short)
+        ));
+
+        // Drop a key byte: the last entry no longer fits.
+        let mut bytes = make_log(vec![key(1)]).encode().as_bytes().to_vec();
+        bytes.pop();
+        assert!(matches!(
+            EncodedAccountLog::parse(bytes),
+            Err(AccountLogError::Short)
+        ));
+    }
+
+    #[test]
+    fn parse_rejects_trailing_bytes() {
+        let mut bytes = make_log(vec![key(1)]).encode().as_bytes().to_vec();
+        bytes.push(0);
+        assert!(matches!(
+            EncodedAccountLog::parse(bytes),
+            Err(AccountLogError::Trailing)
+        ));
+    }
+
+    #[test]
+    fn parse_rejects_missing_domain() {
+        let payload = make_log(vec![]).encode();
+        let without_domain = payload.as_bytes()[ACCOUNT_LOG_DOMAIN.len()..].to_vec();
+        assert!(matches!(
+            EncodedAccountLog::parse(without_domain),
+            Err(AccountLogError::Domain)
+        ));
+    }
+
+    #[test]
+    fn parse_rejects_bad_version_and_tag() {
+        let mut bytes = make_log(vec![]).encode().as_bytes().to_vec();
+        bytes[ACCOUNT_LOG_DOMAIN.len() - 2] = b'9'; // the version character
+        assert!(matches!(
+            EncodedAccountLog::parse(bytes),
+            Err(AccountLogError::Version(v)) if v == "9"
+        ));
+
+        let mut bytes = make_log(vec![key(1)]).encode().as_bytes().to_vec();
+        bytes[ACCOUNT_LOG_DOMAIN.len() + HEADER] = 77; // first entry's tag byte
+        assert!(matches!(
+            EncodedAccountLog::parse(bytes),
+            Err(AccountLogError::Tag(77))
+        ));
+    }
+
+    /// Well-formed bytes carrying an invalid log (a self-referencing remove)
+    /// are rejected at parse: broken logs never get past the boundary. Such
+    /// bytes cannot be produced through the API, so they are handcrafted.
+    #[test]
+    fn parse_rejects_invalid_log() {
+        let mut bytes = ACCOUNT_LOG_DOMAIN.to_vec();
+        bytes.extend_from_slice(&1u32.to_le_bytes());
+        bytes.push(TAG_REMOVE);
+        bytes.extend_from_slice(&0u32.to_le_bytes()); // Remove{0} at position 0
+        assert!(matches!(
+            EncodedAccountLog::parse(bytes),
+            Err(AccountLogError::InvalidRemove { .. })
+        ));
+    }
+
+    /// Full happy path: sign with the account key, verify under the account key.
+    #[test]
+    fn verify_accepts_well_formed_log() {
+        let account_key = Ed25519SigningKey::generate();
+        let account_pub = account_key.verifying_key();
+        let log = make_log(vec![key(1), key(2)]);
+
+        let payload = log.encode();
+        let signed = SignedAccountLog {
+            signature: account_key.sign(payload.as_bytes()),
+            payload,
+        };
+
+        assert_eq!(verify_log(&account_pub, &signed).unwrap(), log);
+    }
+
+    /// A log validly signed by account A, served as the answer to a query for
+    /// account B, fails: B's key does not verify A's signature. This is the
+    /// anti-substitution guarantee.
+    #[test]
+    fn verify_rejects_wrong_account() {
+        let account_key = Ed25519SigningKey::generate();
+        let payload = make_log(vec![]).encode();
+        let signed = SignedAccountLog {
+            signature: account_key.sign(payload.as_bytes()),
+            payload,
+        };
+
+        let other = Ed25519SigningKey::generate().verifying_key();
+        assert!(matches!(
+            verify_log(&other, &signed),
+            Err(AccountLogError::SignatureInvalid)
+        ));
+    }
+
+    /// A signature over one entry list does not verify another.
+    #[test]
+    fn verify_rejects_swapped_payload() {
+        let account_key = Ed25519SigningKey::generate();
+        let account_pub = account_key.verifying_key();
+
+        let signature = account_key.sign(make_log(vec![key(1)]).encode().as_bytes());
+        let signed = SignedAccountLog {
+            payload: make_log(vec![key(2)]).encode(),
+            signature,
+        };
+        assert!(matches!(
+            verify_log(&account_pub, &signed),
+            Err(AccountLogError::SignatureInvalid)
+        ));
+    }
+
+    /// Appending entries is an extension; anything else is stale or a fork.
+    #[test]
+    fn extension_accepts_appends_only() {
+        let old = make_log(vec![key(1)]).encode();
+        let new = make_log(vec![key(1), key(2)]).encode();
+        verify_extension(&old, &new).unwrap();
+
+        // Same length: stale, even with identical contents.
+        assert!(matches!(
+            verify_extension(&old, &old),
+            Err(AccountLogError::NotLonger { old: 1, new: 1 })
+        ));
+
+        // Shrinking: stale.
+        assert!(matches!(
+            verify_extension(&new, &old),
+            Err(AccountLogError::NotLonger { old: 2, new: 1 })
+        ));
+
+        // Longer but rewrites entry 0: fork.
+        let fork = make_log(vec![key(3), key(2)]).encode();
+        assert!(matches!(
+            verify_extension(&old, &fork),
+            Err(AccountLogError::Forked)
+        ));
+    }
+}
diff --git a/core/account/src/error.rs b/core/account/src/error.rs
new file mode 100644
index 0000000..bb5a846
--- /dev/null
+++ b/core/account/src/error.rs
@@ -0,0 +1,36 @@
+pub use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum AccountError {
+    #[error("Generic: {0}")]
+    Generic(String),
+    #[error("No account entry for id: {0} ")]
+    MissingEntry(String),
+    #[error("invalid account address")]
+    InvalidAddress,
+}
+
+/// Failures decoding, verifying, or replaying an account log.
+#[derive(Debug, Error)]
+pub enum AccountLogError {
+    #[error("payload shorter than its declared layout")]
+    Short,
+    #[error("payload is missing the account-log domain prefix")]
+    Domain,
+    #[error("unsupported log version {0}")]
+    Version(String),
+    #[error("unknown entry tag {0}")]
+    Tag(u8),
+    #[error("payload has bytes past its declared entries")]
+    Trailing,
+    #[error("text entry is not valid UTF-8")]
+    Utf8,
+    #[error("account signature verification failed")]
+    SignatureInvalid,
+    #[error("remove at position {position} does not point at an earlier live add ({index})")]
+    InvalidRemove { position: usize, index: u32 },
+    #[error("new log does not have more entries than the old ({new} <= {old})")]
+    NotLonger { old: u32, new: u32 },
+    #[error("new log rewrites history instead of extending it (fork)")]
+    Forked,
+}
diff --git a/core/account/src/lib.rs b/core/account/src/lib.rs
index 1b90c8c..794417f 100644
--- a/core/account/src/lib.rs
+++ b/core/account/src/lib.rs
@@ -1,13 +1,22 @@
+#[cfg(feature = "dev")]
+mod account;
+mod addr;
+mod codec;
 mod directory;
+mod error;
+mod account_log;
 
+use crypto::Ed25519VerifyingKey;
 pub use directory::{
     AccountDirectory, BUNDLE_VERSION, BundleError, DecodedBundle, DeviceId, DeviceSet, Lamport,
     ResolveError, SignedDeviceBundle, decode_bundle_payload, encode_bundle_payload,
     resolve_device_ids, verify_bundle,
 };
+pub use codec::{ACCOUNT_LOG_DOMAIN, verify_extension, verify_log};
+pub use error::{AccountError, AccountLogError};
+pub use account_log::{AccountEntry, AccountLog, EncodedAccountLog, EntryData, SignedAccountLog};
+pub use addr::AccountAddr;
 
 #[cfg(feature = "dev")]
-mod account;
+pub use account::TestLogosAccount;
 
-#[cfg(feature = "dev")]
-pub use account::{AddDelegateSignerError, TestLogosAccount};