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Also known as: CAST-128, CAST-256, CAST5
A family of symmetric-key block ciphers, notably CAST-128 and CAST-256, used in applications like PGP.
DESCRIPTION
CAST refers to a family of symmetric block ciphers developed by Carlisle Adams and Stafford Tavares, including CAST-128 (CAST5) and CAST-256. CAST-128 is a 64-bit block cipher with variable key sizes up to 128 bits and was standardized in RFC 2144. CAST-256 extended the design to 128-bit blocks and larger keys during the AES competition era, though it was not selected. CAST ciphers saw practical use in software such as PGP and certain IPsec configurations. While secure when properly implemented, CAST has been eclipsed by AES for performance and standardization reasons. Legacy systems may still require CAST for compatibility, and secure modes of operation remain essential.
ALTERNATIVE DEFINITIONS
IETF
CAST-128 is a symmetric-key block cipher with a 64-bit block size and a variable key length from 40 to 128 bits.
Specifies block size and variable key length as standardized in RFC 2144.
Wikipedia
CAST-128, also known as CAST5, is a symmetric-key block cipher used in some GnuPG and PGP implementations.
Emphasizes real-world use in PGP/GnuPG ecosystems.
RELATED TERMS
Symmetric Block Cipher Algorithms Pretty Good Privacy Cipher Block Chaining Key Schedule Feistel Function RFC 2144 OpenPGP
COMPETING TERMS
Rijndael International Data Encryption Algorithm
RELATIONS
- Broader: Symmetric Block Cipher Algorithms
KNOWLEDGE AREAS
Data Security Software Engineering
TARGET AUDIENCE
Security Architect Cybersecurity Engineer Software Developer Chief Information Security Officer
EXTERNAL SOURCES
- Wikipedia - CAST-128
- IETF - Official specification for CAST-128
- IETF - ESP CBC-mode cipher algorithms including CAST
- OpenSSL - OpenSSL CAST5 cipher usage
TOP QUESTIONS
What does CAST stand for in cryptography?
CAST comes from the designers’ names, Carlisle Adams and Stafford Tavares. It denotes a family of symmetric block ciphers, notably CAST-128 and CAST-256.
Category: Origin
Is CAST still used in modern systems?
CAST appears primarily in legacy contexts such as PGP archives and some IPsec deployments; AES dominates modern standards and hardware acceleration.
Category: Relevancy
How does CAST-128 differ from CAST-256?
CAST-128 uses 64-bit blocks with keys up to 128 bits, while CAST-256 uses 128-bit blocks with larger key sizes and more rounds.
Category: Comparison
What attacks target CAST ciphers?
Reduced-round variants have academic attacks, but full-round CAST-128 remains secure in practice when used with safe modes and strong keys.
Category: Risks
What modes should be used with CAST?
Avoid ECB. Use CBC, CTR, or authenticated modes; combine with authentication (e.g., HMAC) or use AEAD constructions when available.
Category: BestPractices
Why was CAST not selected as AES?
While strong, CAST-256 did not achieve the same performance, simplicity, or consensus as Rijndael, which became AES.
Category: History
Where can I find a standard for CAST-128?
IETF RFC 2144 documents CAST-128’s specification and is a primary open reference.
Category: Resources
What is a competing term for CAST in modern systems?
AES is the predominant alternative due to NIST standardization and broad hardware support.
Category: Alternatives
Is CAST suitable for compliance-driven environments?
Compliance typically favors standardized algorithms like AES; CAST may be acceptable for legacy decryption but not for new encryption.
Category: Compliance