رمزنگاری و امضای دیجیتالی

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Presentation transcript:

رمزنگاری و امضای دیجیتالی فصل چهارم رمزنگاری و امضای دیجیتالی

رمز سزار (Caesar) A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 t h i s a e c r m g . 19 7 8 18 26 4 2 17 12 6 27 Key = 5 24 12 13 23 3 5 9 7 22 17 11 4 Y M N X D F J H W R L E

Symmetric Cryptography (رمزنگاری متقارن) Message (Example: Hello) Message Common key کلید مشترک بین A و B A B Encrypted Message (Example: GH&$ALJK#!@) Encrypt رمزگذاری Decrypt رمزگشایی شنود 3DES, DES, AES, RC2, RC5 Problems: Alice and Bob must agree on the secret key without anyone else finding out Anyone who intercepts the key in transit can later read, modify, and forge all messages encrypted using that key

Asymmetric Cryptography (رمزنگاری نامتقارن) Message (Example: Hello) Message Public key کلید عمومی B Private key کلید خصوصی B A B Encrypted Message (Example: GH&$ALJK#!@) Encrypt رمزگذاری Decrypt رمزگشایی شنود RSA, DSA, ECC, IDEA Problems: Key exchange has to be done in a secure way Encryption and decryption are extremely SLOW

(Example: GH&$ALJK#!@ , GH%$A) رمزنگاری ترکیبی Message (Example: Hello) Message Session key کلید نشست A B تولید کلید متقارن Encrypted Message (Example: GH&$ALJK#!@ , GH%$A) رمزگذاری پیغام با کلید نشست رمزگشایی پیغام با کلید نشست شنود رمزگذاری کلید نشست با کلید عمومی B رمزگشایی کلید نشست با کلید خصوصی B

Hash Functions It was the best of times, it was the worst of times It was the best of thymes, it was the worst of times Small Difference 3au8 e43j jm8x g84w Hash Function b6hy 8dhy w72k 5pqd Hash Function Large Difference Examples: MD5 (128 bit), SHA-1 (160 bit)

Signature & Verification Hashing + Encryption = Signature Creation Message (Example: Hello) Transmitted Message Example: Hello , fg%#e%@d12A Signature B A Message Digest Hash Function Expected Digest Decrypt رمزگشایی با کلید عمومی A Message Digest Hash Function Encrypt رمزگذاری با کلید خصوصی A Signature If these are the same, then the message has not changed Hashing + Decryption = Signature Verification