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2. Terms and Acronyms

  • AEAD - Authenticated Encryption with Additional Data

  • Authentication Server, cdoc2-auth - SID/MID. Generates session tokens to presented to other CDOC2 SID/MID infrastructure servers.

  • Authentication ticket - SID/MID. A server-specific presentation of the CDOC2 Authentication Token, created by selectively disclosing only the nonce relevant to one CSS server. One Authentication Token is signed once by the Recipient; a separate Authentication Ticket is derived and sent to each CSS.

  • Capsule - Data structure, which contains encryption scheme-specific information (encrypted symmetric keys, public keys, salt, server object references, ...)
    which Recipient can use to derive, establish or retrieve decryption keys for decrypting the CDOC2 Container. Capsule can either be a Server Capsule, a Container Capsule or a Shares Capsule.

  • CC - CDOC2 Capsule. Data structure inside CDOC2 Container. CC contains information for decrypting the payload of CDOC2 Container.
    That information could be a symmetric cryptographic key, a share of symmetric cryptographic key,
    or necessary data for establishing such key with key derivation algorithm or key-agreement protocol, for example, with ECDH.

  • CDOC - Crypto Digidoc, encrypted file transmission format used in the Estonian eID ecosystem.

  • CDOC 1.0 - Unofficial term for all (XML-ENC based) CDOC formats preceding this specification.

  • CDOC2 Authentication Token - SID/MID. Credential constructed by the Client containing CSS server nonces, signed by the Recipient via SID/MID. Presented to CSS servers to authorize Key Share downloads for a specific CDOC2 Container. Encoded as an SD-JWT. Valid for 300 seconds.

  • CDOC2 Client Application - Software used by Sender and Recipient to create and decrypt CDOC2 Containers, interact with CCS and CSS servers, and handle eID authentication.

  • ChaCha20-Poly1305 - Authenticated encryption algorithm (AEAD) used to encrypt the payload of a CDOC2 Container. Uses a 256-bit CEK and a 96-bit nonce.

  • CDOC2 Container - File format for transmitting the encrypted payload and metadata information,
    including the capsule from Sender to Recipient.

  • CDOC2 System - IT system, which allows users to send encrypted files to each other with the help of CDOC2 Client Applications and CDOC2 Capsule Servers.

  • CCS - CDOC2 Capsule Server, which mediates CDOC2 Server Capsules between Sender and Recipient.

  • CEK - Content Encryption Key. Symmetric key used to encrypt the payload of CDOC2 Container.

  • Container Capsule - A Capsule that is created inside a CDOC2 Container and is therefore not sent to a CDOC2 Capsule Server.

  • CSPRNG - Cryptographically Secure Pseudo-Random Number Generator. Used throughout the system to generate key material such as the FMK, ephemeral key pairs, KEK shares, and salts.

  • CSS - SID/MID. CDOC2 Shares Server, which mediates Key Shares between Sender and Recipient.

  • ECC - Elliptic-Curve Cryptography

  • ECC DH - Elliptic-Curve Cryptography Diffie Hellman key-establishment algorithm

  • ECC CDH - Elliptic-Curve Cryptography Co-factor Diffie Hellman key-establishment algorithm

  • ECDH - Elliptic-curve Diffie–Hellman. Key-agreement protocol that allows two parties, each having an EC public–private key pair, to establish a shared secret over an insecure channel.

  • eID - Electronic identity. An umbrella term for digital identification means such as the Estonian ID-card, Mobile-ID, and Smart-ID.

  • FMK - File Master Key. Cryptographic key material for deriving other encryption and HMAC keys.

  • hardware security token - Smart-card (for example, the Estonian eID ID-card) or FIDO authenticator with asymmetric cryptographic keys.

  • HHK - Header HMAC Key.

  • HMAC - Hash-Based Message Authentication Code. Protects the integrity of the CDOC2 Container.

  • HKDF - HMAC-based Key Derivation Function (RFC 5869). Used to derive CEK, HHK, and KEK from the FMK.

  • KEK - Key Encryption Key. Symmetric key used to encrypt (wrap) the FMK, so that the FMK could be transmitted inside the CDOC2 Container to Recipient.

  • Key Share - SID/MID. Key Shares are created by splitting cryptographic material required for encrypting/decrypting a CDOC2 document. These are stored inside Shares Capsules. Key Shares are always distributed among different Shares Servers and depending on the encryption scheme, all or a certain number of shares are needed to construct the original key value.

  • KTS - Key Transfer Server. Umbrella term used in use case IDs (UC.KTS.xx) covering both the CDOC2 Capsule Server (CCS) and the CDOC2 Shares Server (CSS), as both mediate transfer of key material between Sender and Recipient.

  • nonce - A randomly generated, single-use value issued by a CSS server, used to prevent replay of authentication tokens.

  • PBKDF2 - Password-Based Key Derivation Function 2 (RFC 2898). Used to derive key material from a pre-shared password in symmetric key encryption schemes.

  • Recipient - The party who receives a CDOC2 Container and decrypts its payload using their key material (e.g. ID-card private key, pre-shared secret, or Key Shares from CSS servers).

  • RP Server, cdoc2-rp - SID/MID. Relying Party Server. Mediates Smart-ID RP API and Mobile-ID REST API calls to SK services

  • RSA - Rivest–Shamir–Adleman. Asymmetric encryption algorithm supported as a recipient key type in CDOC2 (used in schemes SC02 and SC04) via RSA-OAEP.

  • SD-JWT - Selective Disclosure JSON Web Token. Token format that supports selective disclosure of claims. Used as the encoding format for Session Token and CDOC2 Authentication Token.

  • Sender - The party who creates a CDOC2 Container, encrypts the payload, and distributes Capsules to one or more Recipients.

  • Server Capsule - A Capsule that is mediated by a CDOC2 Capsule Server.

  • Session Token- SID/MID. Short-lived credential issued by cdoc2-auth to the Client upon successful MID/SID authentication. Presented to CDOC2 infrastructure servers to prove Recipient identity. Valid for 24 hours. Encoded as an SD-JWT.

  • Shares Capsule - Encryption/decryption key material which is split into Key Shares in order to distribute it to multiple CSS servers.