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Apr
27
comment How solid is this method of encryption?
Requests for analysis of amateur cryptographic schemes are off-topic here, so expect the question to be closed (no hard feelings). Also, while I cannot point out any specific problem with this approach, I am pretty sure it isn't secure. Good references for your further research include how to publish a cipher concept and lessons learned and common misconceptions about cryptography.
Apr
22
comment How secure is this logarithmic encryption algorithm?
Semi-Related: How to publish a cipher (concept) has a number of hints on what to do if you want to design and evaluate your own ciphers.
Apr
21
comment GCM: use the same key:IV pair to encrypt file and filename
You could derive the new IV from the old one (e.g. by hashing, using a KBKDF, ...) and still be safe, it only matters that they are different. That way, you would not have to save two different IVs.
Mar
29
comment I homebrewed an algorithm. Is it secure?
See How to publish a cipher for some pointers on what to do when developing your own ciphers, it has some good pointers.
Mar
26
comment Given the first half of a SHA-1 hash, can you predict the second half?
@MaartenBodewes Yes. This wasn't to suggest that this should be closed as duplicate, only to give a pointer to existing knowledge on the site for this topic.
Mar
23
comment Authenticated encryption with Signatures
As this was just released a few days ago, you may want to take a look at the iMessage attack, which works exactly because the difference between MAC and signatures.
Mar
23
comment Unlinkability of Merkle-Damgård hash function results
@MaartenBodewes I just added some additional info at the bottom of the question - it may be that this question is an open research question. If that is indeed the case, feel free to withdraw the bounty.
Mar
23
comment Unlinkability of Merkle-Damgård hash function results
@MaartenBodewes Thank you for the attempt, and for the bounty!
Mar
23
comment Unlinkability of Merkle-Damgård hash function results
@Biv: I also proposed a tag wiki for the unlinkability tag, which is waiting for review now. Thanks for pointing this out to me.
Mar
23
comment Unlinkability of Merkle-Damgård hash function results
@Biv: Thank you for your edit. However, the equation you wanted to edit in did not match what I wanted to achieve (at least in my understanding). I edited your edit to clarify things further and hope that it is now clear what my goal is.
Mar
23
comment Should I use a self-designed Block Cipher Mode of Operation?
Also read this question and answer, which deals with self-designed crypto algorithms but also applies to modes of operation.
Mar
22
comment A simple keyed message authentication code using XOR
Your authentication system should also authenticate who the message is from. The adversary is not interested in finding a second tag for your message, but a new one for his own. If your message is "pay 100 USD", an attacker could change it to "pay 1000 USD" and compute a valid authentication tag over the new message. To the recipient, this would be indistinguishable from you sending that 1000 USD message.
Mar
22
comment Combining public key signature schemes and private key encryption
I'm not quite clear on what goal you want to achieve with this. Can you clarify what your combination should achieve and what your exact question is?
Mar
22
comment Which compound algorithms would weaken cipher, not strengthen it?
Disclaimer: Why I have a background in IT Security and encryption, I am not enough of an expert to give you a 100% certain answer. Take all my replies with a grain of salt and properly analyze any construct you may choose to build, just in case. Also see, for example, this question and answers about common mistakes made in cryptography.
Mar
22
comment Which compound algorithms would weaken cipher, not strengthen it?
I moved my comments into their own answer and deleted them here.
Mar
22
comment Signing MD5 hashes with RSA?
Thank you for the info, I had indeed misunderstood things. I added PSS and replaced the explanation about md5 with a pointer to your comments and the other answer.
Mar
22
comment Given the first half of a SHA-1 hash, can you predict the second half?
While the question may not be a duplicate, the answer is the same as the one given to this question
Mar
21
comment RSA: Recover e given factorization of N and plaintext/ciphertext pair
You probably mean 2048-bit and not "2-48-bit", right?
Mar
21
comment Sodium crypto_aead_aes256gcm_encrypt ciphertext length > plaintext length
The API of sodium looks like it will take arbitrary lengths of plaintext using the MESSAGE and MESSAGE_LEN parameters, so you should be good. Don't forget to mark the answer as accepted to clean up the list of unanswered questions :)
Mar
21
comment Sodium crypto_aead_aes256gcm_encrypt ciphertext length > plaintext length
(If you manually split the file into chunks of 16 byte and encrypt them separately, THEN you will double the file size. However, there's no good reason to do this instead of encrypting the whole file at once)