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A cryptographic transcript is the specified record of messages exchanged during a cryptographic protocol execution. Its content depends on the purpose: a public communication record differs from a participant's complete private view, which can also include its input, secret state and randomness. Order, message boundaries and the identities or roles associated with messages can matter.

For a concrete standardized use, Transport Layer Security 1.3 authenticates a transcript hash computed from selected handshake messages in their prescribed order. It includes handshake type and length headers, but excludes record-layer headers. Its retry path specifies a special replacement for the first client message.^[ref]

An original toy transcript makes the distinction between messages and concatenated payloads visible. Suppose a sender transmits ab and then c. The payload sequence is $(\mathtt{ab},\mathtt{c})$. Another execution transmits a and then bc. Both flatten to ASCII bytes 61 62 63, even though they have different first messages. A length-prefixed encoding preserves the distinction as 02 61 62 01 63 versus 01 61 02 62 63. A hash cannot recover boundaries that were omitted before hashing.

Direction can be equally important. The records “initiator sends yes” and “responder sends yes” contain identical payloads but assert different behavior. A format used to authenticate those assertions must bind the relevant roles, either explicitly in the encoded record or through an unambiguous protocol context. This toy rule is not a replacement for the TLS transcript format.

In a security proof, the observer's view and its correlations must be specified. A cryptographic simulator that samples messages with correct individual frequencies may still produce the wrong joint probability distribution with the cryptographic environment's retained inputs. A cryptographic transcript also discloses only the observations it records: a transcript omitting timing cannot by itself justify a security claim against timing observations.

A transcript hash is a compact fingerprint of the encoded record under the relevant cryptographic hash function assumptions. An unkeyed hash alone does not identify the speaker: a digital signature or message authentication code must bind it to an authenticated context when that is the intended guarantee.

Incremental hashing of a cryptographic transcript supplies successive encoded message bytes to one continuing cryptographic hash function state. Finalizing after the selected messages gives the same transcript hash as hashing their full concatenation in one call. TLS explicitly permits implementing the handshake computation using a running hash.^[ref]

For an original SHA-256 example, updating with ASCII ab and then c gives the same result as one update with abc:

ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad.

A chunk boundary is an input-delivery detail. It does not insert a separator byte or end a message. A continuing implementation must preserve the processed bit count and any incomplete block as well as its chaining state; final padding encodes the complete input length. SHA-256's specification processes 512-bit blocks and pads the complete message before its final compression operations.^[ref]

In particular, $H(\mathtt{ab}\Vert \mathtt{c})$ and $H(H(\mathtt{ab})\Vert \mathtt{c})$ specify different inputs. The latter inserts a completed digest, with its already finalized hashing boundary, where the original bytes belonged. It is not an incremental implementation of the former.

The protocol must still define which bytes enter the state. TLS includes handshake headers in the transcript hash and specifies a special first-message substitution after a retry. A running hash implements that prescribed input, rather than defining the transcript by itself.^[ref] An unkeyed digest also supplies no sender authentication by itself; the protocol must bind it through its specified digital signature or message authentication code.