Silent Phone

The foundation of Silent Phone is the ZRTP key agreement protocol. Designed by the creator of PGP, Phil Zimmermann, ZRTP provides true peer-to-peer key negotiation and management for secure VOIP communication. Visit the ZRTP FAQ


  • Elliptic Curve Cryptography (P-384)
  • Advanced Encryption Standard (AES-256)
  • Secure Hash Algorithm (SHA-256)
  • Short Authentication String (SAS)
  • Perfect Forward Secrecy
  • Key Continuity

ZRTP uses only FIPS-approved algorithms in all relevant categories to meet the FIPS-140 validation requirements set by NIST FIPS PUB 140-2 Annex A and NIST FIPS PUB 140-2 Annex D, ZRTP is compliant with:

Silent Text

Silent Text utilizes the Silent Circle Instant Messaging Protocol (SCIMP) to provide secure communication over the Jabber XMPP instant message transport. SCIMP was designed in direct relation to ZRTP.


  • Elliptic Curve Diffie–Hellman (ECDH), NIST 800-56A
  • Counter with CBC-MAC (CCM), NIST 800-38C
  • Key Derivation, NIST 800-108
  • Secure Hash Standard, FIPS 180-4
  • Skein Hash/MAC (optional)
  • Advanced Encryption Standard (AES), FIPS 197
  • Hash Commitment
  • Short Authentication String
  • Key Continuity
  • Perfect Forward Secrecy

Source Code

Silent Circle believes strongly in transparency of design, particularly in regards to secure communications. As such, all source code for Silent Circle can be viewed at the links below: