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TLS/DTLS AES-CTR draft-ietf-tls-ctr-00

TLS/DTLS AES-CTR draft-ietf-tls-ctr-00. Nagendra Modadugu Eric Rescorla. AES-CTR Overview. Works like a stream cipher, e.g. RC4 XOR keystream with plain text: CT[i] := PT[i]  AES(CTR(i)) Increment Counter Counter encrypted to generate keystream

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TLS/DTLS AES-CTR draft-ietf-tls-ctr-00

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  1. TLS/DTLS AES-CTRdraft-ietf-tls-ctr-00 Nagendra Modadugu Eric Rescorla

  2. AES-CTR Overview • Works like a stream cipher, e.g. RC4 • XOR keystream with plain text: CT[i] := PT[i]  AES(CTR(i)) • Increment Counter • Counter encrypted to generate keystream • Counter MUST never be re-used (with same key) • No harm if Counter is public • But MUST be initially unpredictable

  3. Motivation • Low bandwidth • Save between 17-32 bytes compared to CBC • Random access (for DTLS) • Parallelizable/pipelining • Implement both block/stream ciphers with AES

  4. Counter Design [1] IV • IV := {client_write_IV, server_write_IV} • SEQ := {seq_num}(64-bits) • BLOCK_CTR := 1 (16-bits) IV SEQ SEQ SEQ BLOCK_CTR 32 bits

  5. Counter Design [2] • IV’s generated by TLS/DTLS KDF • Refreshed upon session re-negotiation • Sequence number • Implicit for TLS • For DTLS, use (epoch || seq_num) • Block counter • 16-bits plenty for TLS/DTLS records

  6. Questions Received • Number of IV bits • 48-bits sufficient? • Editorial comments • Security considerations section

  7. Questions?

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