Sha1 checksum5/27/2023 ![]() This way if I sign something with my key, you can know for sure it was me. A signature created with the private key can be verified by the public key, but the public key can't be used to create signatures. Signatures use asymmetric cryptography, so there is a public key and a private key. This is important, because while the hash for a file can be calculated by anyone, a signature can only be calculated by someone who has the secret. ![]() Unlike checksums or hashes, a signature involves a secret. Connor J's answer gives examples for Windows. On Linux you can use the md5sum, sha1sum, sha256sum, etc utilities. The best example of where it makes sense to verify a hash is when retrieving the hash from the software's trusted website (using HTTPS of course), and using it to verify files downloaded from an untrusted mirror. They may be able to create an innocent change in the original that causes it to have the same hash as a malicious file, which they could then send you. Using a hash that isn't collision resistant may be problematic if your adversary can modify the legitimate file (for example, contributing a seemingly innocent bug fix). If an attacker is able to modify files on that site or intercept and modify your connection, they can simply substitute the files for malicious versions and change the hashes to match. Retrieving the hash from the same site you're downloading the files from doesn't guarantee anything. If you plan to use a hash to verify a file, you must obtain the hash from a separate trusted source. Using a cryptographic hash to verify integrity SHA256 is commonly used today, and is safe against both. MD5 and SHA1 are both broken in regard to collisions, but are safe against preimage attacks (due to the birthday paradox collisions are much easier to generate). Collision resistance means that it isn't feasible to create two files that have the same hash, and preimage resistance means that it isn't feasible to create a file with the same hash as a specific target file. What is a cryptographic hash?Ĭryptographic hashes provide additional properties over simple checksums (all cryptographic hashes can be used as checksums, but not all checksums are cryptographic hashes).Ĭryptographic hashes (that aren't broken or weak) provide collision and preimage resistance. Examples of checksums are CRCs, Adler-32, XOR (parity byte(s)). In general a checksum provides no guarantee that intentional modifications weren't made, and in many cases it is trivial to change the file while still having the same checksum. What is a checksum?Ī checksum simply verifies with a high degree of confidence that there was no corruption causing a copied file to differ from the original (for varying definitions of "high"). That would be the easiest solution I can think of.You mention checksums, PGP, and SHA in your question title, but these are all different things. I'm not familiar with the AWS SDK for PHP, but the AWS CLI and other SDKs have a sync function that will check if the object in s3 has changed and only download it if necessary. The only wrinkle is that computing the etag locally can be a PITA, since it's not a normal hash function. Depending on how often the etag matches, this could be a significant savings and speed improvement. ![]() It's not as straightforward as a checksum, but since it's centrally stored in s3, Server B can check if the file changed before downloading the object. S3 tracks it's own checksum of objects, known as the etag. This approach is going to be slower and cost you more processing time. Unfortunately, unless you're storing that checksum somewhere, Server B will need to download the object, re-compute the sha1, and compare it to the previous version of the object stored locally. Moving a file into/out of s3 won't change it's checksum. ![]()
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