Exercises: RSA Encrypt / Decrypt
In this exercise you shall encrypt and decrypt messages using the RSA public-key cryptosystem.
Encrypt Message with RSA-OAEP
You are given a text message and a RSA public key (in PEM format). Write a program to encrypt the message, using the RSA-OAEP encryption scheme (RSA + PKCS#1 OAEP padding).
Input:
First line: the input message
The public key length can be 512 bits, 1024 bits, 2048 bits, 3072 bits or 4096 bits.
Output:
The encrypted message, printed as hex string.
Write your code in programming language of choice.
Sample input:
The above input uses a 512-bit RSA public key and a small plain text message, that can fit inside the key length (after the OAEP padding).
Sample output (for the above input):
Note: the above output should be different at each execution due to the randomness injected by the OAEP padding algorithm.
Decrypt a Message with RSA-OAEP
You are given a RSA-OAEP-encrypted ciphertext (as hex string) and a RSA private key (in PEM format). Write a program to decrypt the message, using the RSA-OAEP encryption scheme (RSA + PKCS#1 OAEP padding).
Input:
First line: the ciphertext (the encrypted message), given as hex string
Output:
Print the decrypted message as plain text
Print
Decryption failed!in case of problem
Write your code in programming language of choice.
Sample input:
The above input uses a 512-bit RSA private key and an encrypted ciphertext of the same length.
Sample output (for the above input):
Another sample input (wrong 512-bit private key):
The corresponding output should be:
Note that the RSA-OAEP padding algorithm has built-in checksum, which allows to detect incorrect decryption attempts, but it is not an authenticated encryption scheme.
* Implement Hybrid Encryption / Decryption with RSA-KEM
Write a program to encrypt a large message (bigger than the RSA key length, e.g. a PDF document) using the RSA-KEM hybrid encryption scheme with AES symmetric encryption (use block mode of choice, e.g. GCM or CTR).
Hint:
Check this example first: https://github.com/digitalbazaar/forge#rsakem.
Note that in some languages it is hard to find and RSA-KEM implementation, so you can skip this exercise or use another hybrid encryption scheme (e.g. RSA + AES + HMAC).
Input:
The message for encryption
RSA public key (in PEM format)
Output:
The encrypted ciphertext (hex string)
The random IV salt for the AES cipher (hex string)
The authentication tag / MAC for the encrypted message (hex string)
The encapsulated secret key for the AES algorithm (hex string)
Write a program to decrypt given encrypted message, produced by the previous exercise, using the RSA-KEM hybrid encryption scheme with AES symmetric encryption (use block mode of choice, e.g. GCM or CTR).
Input:
The encrypted ciphertext (hex string)
The random IV salt for the AES cipher (hex string)
The authentication tag / MAC for the encrypted message (hex string)
The encapsulated secret key for the AES algorithm (hex string)
Output:
The decrypted original plaintext message
Print
Decryption failed!if the message decryption is not successful (e.g. wrong password)
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