Swarm provides methods for allowing applications to obtain and validate integrity guarantees on the stored data. In this context, integrity is an independently verifiable guarantee that the data returned for a given name or UUID is exactly the same data that was stored using that name or UUID, perhaps many months or years in the past. This is done by hashing the data using a cryptographic hash algorithm.

Content metadata is not included in the hash. If the application stores the name or UUID and its associated hash value, these can be used later to verify the content has not changed, either through accidental or malicious means.

Integrity seals

An integral seal is a URL containing the object name or UUID, its hash value, and the type of hash algorithm that was used for the computations.

An application can request an integrity seal when it performs a WRITE by including a hashtype query string.

POST http://company.cluster.com/?hashtype=md5 HTTP/1.1

These are the current allowable hash types:

• md5
• sha1
• sha256
• sha384
• sha512

After creating the object and assigning a name or UUID, Swarm replies with a 201 (Created) response that includes a location header with a URL that can later be used to retrieve the data.
In addition to the host and name or UUID, the URL includes the hash type and value computed from the content object. This URL, including the triple name or UUID, hash type, and hash, is known as the content object integrity seal.

Location: http://129.69.251.143/41A140B5271DC8D22FF8D027176A0821
	?hashtype=md5
	&hash=7A25E6067904EAC8002498CF1AE33023

Validating reads

An integrity seal can be used in a subsequent READ request to validate the data stored in a storage cluster (any cluster). By supplying the URL returned in the Location header from the WRITE request (perhaps replacing the host address if connected to a different cluster or node), the application can ask Swarm to validate while reading the data.

GET http://129.69.251.143/41A140B5271DC8D22FF8D027176A0821
	?hashtype=md5
	&hash=7A25E6067904EAC8002498CF1AE33023 HTTP/1.1

When Swarm receives such a READ request, it recomputes the hash of the stored content using the supplied hash type and compares the computed hash with the hash value in the integrity seal.

• Match - If the content was not modified or corrupted in any way, the hashes match. Swarm returns the object with the computed digest as a trailing Location header.
• No match - If the two values do not match, Swarm will drop the connection before sending the object content at the end of the request. 

Because the hash algorithms are published and well-known, users and third parties can independently validate an object that was stored by Swarm by reading its contents, computing the hash value, and comparing it with the hash value in the seal. By publishing an integrity seal when it is created, you can ensure that the stored content was not modified and it has always been associated with the same UUID.

Application-initiated hash upgrading

Occasionally, cryptographers and mathematicians may defeat a cryptographic algorithm, making it possible for hackers to generate different content that has exactly the same hash value as previously-stored content. This issue occurred with the md5 and sha1 algorithms, but not the sha256, sha384, or sha512 algorithms.

Unlike other fixed content storage solutions, Swarm allows a user or application to upgrade a hash algorithm for an existing individual integrity seal. This is done by issuing a READ request with the name or UUID, the current hash type and hash, and then specifying a different, presumably stronger, hash type in the newhashtype query parameter.

GET http://129.69.251.143/41A140B5271DC8D22FF8D027176A0821
	?hashtype=md5
	&hash=7A25E6067904EAC8002498CF1AE33023
	&newhashtype=sha256 HTTP/1.1

This READ request will first validate the given integrity seal, then reseal it by wrapping the content in the new, upgraded hash algorithm – sha256 in the example. If the requested object fails to validate against its integrity seal, Swarm will send a 200 OK response but will drop the connection prior to sending the object content. If the object validates properly, a new integrity seal will be returned with the new hash type and hash value.