Key points to remember about cryptography applied to evidence
- Key points to remember about cryptography applied to evidence
- A hash function transforms data into a fixed-size hash value; it is designed to operate in one direction.
- The hash serves as a content identifier: if the document changes, the hash changes.
- Asymmetric cryptography relies on a private key/public key pair: the private key signs, the public key verifies.
- A digital signature and sealing are analysed as mechanisms ensuring verifiable integrity and origin, based on hashing, keys and signature algorithms.
What is a hash function?
A hash function is an algorithm that takes input data of arbitrary size and produces a fixed-size output value (commonly referred to as a message digest). In NIST standards, “approved” hash functions (SHA-2, including SHA-256) are described as one-way functions intended to produce a condensed representation of a message.
From an evidential perspective, the relevant technical point is as follows: the hash function does not disclose the content itself, but it enables verification of content identity by recomputation, provided that the same algorithm and calculation rules are used (this point becomes sensitive where format transformations occur).
What is a hash (digest)?
The hash (or digest) is the value produced by a hash function. It operates as a computed summary of a document: if a single bit changes, the hash is expected to change in an unpredictable manner. This explains why, in practice, the hash is often timestamped or signed rather than the full file: the hash is concise, stable and suitable for verification.
In an evidential file, the hash performs the function of a technical index: it allows a proof mechanism (signature, seal, timestamp, log) to be linked, without ambiguity, to a specific version of a document.-ce qu’une empreinte (hash) ?Qu’est-ce qu’un algorithme SHA-256 ?
What is the SHA-256 algorithm?
SHA-256 is a hash function within the SHA-2 family, defined in the Secure Hash Standard (FIPS 180-4). It produces a 256-bit hash (32 bytes).
Within evidential chains, SHA-256 is frequently used as a hashing algorithm, notably because it is widely implemented and referenced in security standards. The evidential issue does not lie in “SHA-256 as such”, but in the ability to demonstrate which algorithm has been applied and to recompute the hash in a reproducible manner.
What is asymmetric cryptography?
Asymmetric cryptography (or public-key cryptography) relies on a pair of mathematically related keys: a private key and a public key. It enables, in particular, the generation of signatures that can be verified by third parties without disclosure of the private key.
To situate these mechanisms, certain families of asymmetric algorithms are standardised in reference texts: for example, RSA in PKCS #1 / RFC 8017.
What is a private key and a public key?
The private key is the key that must remain under the exclusive control of its holder: it is used, in particular, to generate a digital signature. The public key is distributed to verifiers: it is used to verify the corresponding signature. The Digital Signature Standard FIPS 186-5 expressly states that only the entity holding the private key can generate a signature, and that verification is carried out using the public key.
In evidential practice, the decisive question then becomes: how can it be demonstrated that a given public key corresponds to a given identity (natural or legal person)? This is precisely the role of certificates (PKI), addressed in a separate glossary.
What is sealing (in the cryptographic sense)?
In a cryptographic context, sealing refers to a process that “closes” a piece of content by linking it to verifiable proof of integrity, so that any alteration becomes detectable. In practice, sealing is generally constructed through a combination of mechanisms: hashing (to identify the content) together with a digital signature or cryptographic authentication mechanism (to attest the operation), sometimes supplemented by timestamping where the date is also at issue.
