Pull in Viget posts
This commit is contained in:
David Eisinger
233
content/elsewhere/lets-make-a-hash-chain-in-sqlite/index.md
Normal file
233
content/elsewhere/lets-make-a-hash-chain-in-sqlite/index.md
Normal file
@@ -0,0 +1,233 @@
|
||||
---
|
||||
title: "Let’s Make a Hash Chain in SQLite"
|
||||
date: 2021-06-30T00:00:00+00:00
|
||||
draft: false
|
||||
needs_review: true
|
||||
canonical_url: https://www.viget.com/articles/lets-make-a-hash-chain-in-sqlite/
|
||||
---
|
||||
|
||||
I\'m not much of a cryptocurrency enthusiast, but there are some neat
|
||||
ideas in these protocols that I wanted to explore further. Based on my
|
||||
absolute layperson\'s understanding, the \"crypto\" in
|
||||
\"cryptocurrency\" describes three things:
|
||||
|
||||
1. Some public key/private key stuff to grant access to funds at an
|
||||
address;
|
||||
2. For certain protocols (e.g. Bitcoin), the cryptographic
|
||||
puzzles[^1^](#fn:1 "see footnote"){#fnref:1 .footnote} that miners
|
||||
have to solve in order to add new blocks to the ledger; and
|
||||
3. The use of hashed signatures to ensure data integrity.
|
||||
|
||||
Of those three uses, the first two (asymmetric cryptography and
|
||||
proof-of-work) aren\'t that interesting to me, at least from a technical
|
||||
perspective. The third concept, though --- using cryptography to make
|
||||
data verifiable and tamper-resistant --- that\'s pretty cool, and
|
||||
something I wanted to dig into. I decided to build a little
|
||||
proof-of-concept using [SQLite](https://www.sqlite.org/index.html), a
|
||||
\"small, fast, self-contained, high-reliability, full-featured, SQL
|
||||
database engine.\"
|
||||
|
||||
A couple notes before we dive in: these concepts aren\'t unique to the
|
||||
blockchain; Wikipedia has good explanations of [cryptographic hash
|
||||
functions](https://en.wikipedia.org/wiki/Cryptographic_hash_function),
|
||||
[Merkle trees](https://en.wikipedia.org/wiki/Merkle_tree), and [hash
|
||||
chains](https://en.wikipedia.org/wiki/Hash_chain) if any of this piques
|
||||
your curiosity. This stuff is also [at the core of
|
||||
git](https://initialcommit.com/blog/git-bitcoin-merkle-tree), which is
|
||||
really pretty neat.
|
||||
|
||||
[]{#onto-the-code}
|
||||
|
||||
## Onto the code [\#](#onto-the-code "Direct link to Onto the code"){.anchor aria-label="Direct link to Onto the code"}
|
||||
|
||||
Implementing a rudimentary hash chain in SQL is pretty simple. Here\'s
|
||||
my approach, which uses \"bookmarks\" as an arbitrary record type.
|
||||
|
||||
``` {.code-block .line-numbers}
|
||||
PRAGMA foreign_keys = ON;
|
||||
SELECT load_extension("./sha1");
|
||||
|
||||
CREATE TABLE bookmarks (
|
||||
id INTEGER PRIMARY KEY,
|
||||
signature TEXT NOT NULL UNIQUE
|
||||
CHECK(signature = sha1(url || COALESCE(parent, ""))),
|
||||
parent TEXT,
|
||||
url TEXT NOT NULL UNIQUE,
|
||||
FOREIGN KEY(parent) REFERENCES bookmarks(signature)
|
||||
);
|
||||
|
||||
CREATE UNIQUE INDEX parent_unique ON bookmarks (
|
||||
ifnull(parent, "")
|
||||
);
|
||||
```
|
||||
|
||||
This code is available on
|
||||
[GitHub](https://github.com/dce/sqlite-hash-chain) in case you want to
|
||||
try this out on your own. Let\'s break it down a little bit.
|
||||
|
||||
- First, we enable foreign key constraints, which aren\'t on by
|
||||
default
|
||||
- Then we pull in SQLite\'s [`sha1`
|
||||
function](https://www.i-programmer.info/news/84-database/10527-sqlite-317-adds-sha1-extension.html),
|
||||
which implements a common hashing algorithm
|
||||
- Then we define our table
|
||||
- `id` isn\'t mandatory but makes it easier to grab the last entry
|
||||
- `signature` is the SHA1 hash of the bookmark URL and parent
|
||||
entry\'s signature; it uses a `CHECK` constraint to ensure this
|
||||
is guaranteed to be true
|
||||
- `parent` is the `signature` of the previous entry in the chain
|
||||
(notice that it\'s allowed to be null)
|
||||
- `url` is the data we want to ensure is immutable (though as
|
||||
we\'ll see later, it\'s not truly immutable since we can still
|
||||
do cascading updates)
|
||||
- We set a foreign key constraint that `parent` refers to another
|
||||
row\'s `signature` unless it\'s null
|
||||
- Then we create a unique index on `parent` that covers the `NULL`
|
||||
case, since our very first bookmark won\'t have a parent, but no
|
||||
other row should be allowed to have a null parent, and no two rows
|
||||
should be able to have the same parent
|
||||
|
||||
Next, let\'s insert some data:
|
||||
|
||||
``` {.code-block .line-numbers}
|
||||
INSERT INTO bookmarks (url, signature) VALUES ("google", sha1("google"));
|
||||
|
||||
WITH parent AS (SELECT signature FROM bookmarks ORDER BY id DESC LIMIT 1)
|
||||
INSERT INTO bookmarks (url, parent, signature) VALUES (
|
||||
"yahoo", (SELECT signature FROM parent), sha1("yahoo" || (SELECT signature FROM parent))
|
||||
);
|
||||
|
||||
WITH parent AS (SELECT signature FROM bookmarks ORDER BY id DESC LIMIT 1)
|
||||
INSERT INTO bookmarks (url, parent, signature) VALUES (
|
||||
"bing", (SELECT signature FROM parent), sha1("bing" || (SELECT signature FROM parent))
|
||||
);
|
||||
|
||||
WITH parent AS (SELECT signature FROM bookmarks ORDER BY id DESC LIMIT 1)
|
||||
INSERT INTO bookmarks (url, parent, signature) VALUES (
|
||||
"duckduckgo", (SELECT signature FROM parent), sha1("duckduckgo" || (SELECT signature FROM parent))
|
||||
);
|
||||
```
|
||||
|
||||
OK! Let\'s fire up `sqlite3` and then `.read` this file. Here\'s the
|
||||
result:
|
||||
|
||||
``` {.code-block .line-numbers}
|
||||
sqlite> SELECT * FROM bookmarks;
|
||||
+----+------------------------------------------+------------------------------------------+------------+
|
||||
| id | signature | parent | url |
|
||||
+----+------------------------------------------+------------------------------------------+------------+
|
||||
| 1 | 759730a97e4373f3a0ee12805db065e3a4a649a5 | | google |
|
||||
| 2 | 64633167b8e44cb833fbfa349731d8a68e942ebc | 759730a97e4373f3a0ee12805db065e3a4a649a5 | yahoo |
|
||||
| 3 | ce3df1337879e85bc488d4cae129719cc46cad04 | 64633167b8e44cb833fbfa349731d8a68e942ebc | bing |
|
||||
| 4 | 675570ac126d492e449ebaede091e2b7dad7d515 | ce3df1337879e85bc488d4cae129719cc46cad04 | duckduckgo |
|
||||
+----+------------------------------------------+------------------------------------------+------------+
|
||||
```
|
||||
|
||||
This has some cool properties. I can\'t delete an entry in the chain:
|
||||
|
||||
`sqlite> DELETE FROM bookmarks WHERE id = 3;`
|
||||
`Error: FOREIGN KEY constraint failed`
|
||||
|
||||
I can\'t change a URL:
|
||||
|
||||
`sqlite> UPDATE bookmarks SET url = "altavista" WHERE id = 3;`
|
||||
`Error: CHECK constraint failed: signature = sha1(url || parent)`
|
||||
|
||||
I can\'t re-sign an entry:
|
||||
|
||||
`sqlite> UPDATE bookmarks SET url = "altavista", signature = sha1("altavista" || parent) WHERE id = 3;`
|
||||
`Error: FOREIGN KEY constraint failed`
|
||||
|
||||
I **can**, however, update the last entry in the chain:
|
||||
|
||||
``` {.code-block .line-numbers}
|
||||
sqlite> UPDATE bookmarks SET url = "altavista", signature = sha1("altavista" || parent) WHERE id = 4;
|
||||
sqlite> SELECT * FROM bookmarks;
|
||||
+----+------------------------------------------+------------------------------------------+-----------+
|
||||
| id | signature | parent | url |
|
||||
+----+------------------------------------------+------------------------------------------+-----------+
|
||||
| 1 | 759730a97e4373f3a0ee12805db065e3a4a649a5 | | google |
|
||||
| 2 | 64633167b8e44cb833fbfa349731d8a68e942ebc | 759730a97e4373f3a0ee12805db065e3a4a649a5 | yahoo |
|
||||
| 3 | ce3df1337879e85bc488d4cae129719cc46cad04 | 64633167b8e44cb833fbfa349731d8a68e942ebc | bing |
|
||||
| 4 | b583a025b5a43727978c169fe99f5422039194ea | ce3df1337879e85bc488d4cae129719cc46cad04 | altavista |
|
||||
+----+------------------------------------------+------------------------------------------+-----------+
|
||||
```
|
||||
|
||||
This is because a row isn\'t really \"locked in\" until it\'s pointed to
|
||||
by another row. It\'s worth pointing out that an actual blockchain would
|
||||
use a [consensus
|
||||
mechanism](https://www.investopedia.com/terms/c/consensus-mechanism-cryptocurrency.asp)
|
||||
to prevent any updates like this, but that\'s way beyond the scope of
|
||||
what we\'re doing here.
|
||||
|
||||
[]{#cascading-updates}
|
||||
|
||||
## Cascading updates [\#](#cascading-updates "Direct link to Cascading updates"){.anchor aria-label="Direct link to Cascading updates"}
|
||||
|
||||
Given that we can change the last row, it\'s possible to update any row
|
||||
in the ledger provided you 1) also re-sign all of its children and 2) do
|
||||
it all in a single pass. Here\'s how you\'d update row 2 to
|
||||
\"askjeeves\" with a [`RECURSIVE`
|
||||
query](https://www.sqlite.org/lang_with.html#recursive_common_table_expressions)
|
||||
(and sorry I know this is a little hairy):
|
||||
|
||||
``` {.code-block .line-numbers}
|
||||
WITH RECURSIVE
|
||||
t1(url, parent, old_signature, signature) AS (
|
||||
SELECT "askjeeves", parent, signature, sha1("askjeeves" || COALESCE(parent, ""))
|
||||
FROM bookmarks WHERE id = 2
|
||||
UNION
|
||||
SELECT t2.url, t1.signature, t2.signature, sha1(t2.url || t1.signature)
|
||||
FROM bookmarks AS t2, t1 WHERE t2.parent = t1.old_signature
|
||||
)
|
||||
UPDATE bookmarks
|
||||
SET url = (SELECT url FROM t1 WHERE t1.old_signature = bookmarks.signature),
|
||||
parent = (SELECT parent FROM t1 WHERE t1.old_signature = bookmarks.signature),
|
||||
signature = (SELECT signature FROM t1 WHERE t1.old_signature = bookmarks.signature)
|
||||
WHERE signature IN (SELECT old_signature FROM t1);
|
||||
```
|
||||
|
||||
Here\'s the result of running this update:
|
||||
|
||||
``` {.code-block .line-numbers}
|
||||
+----+------------------------------------------+------------------------------------------+-----------+
|
||||
| id | signature | parent | url |
|
||||
+----+------------------------------------------+------------------------------------------+-----------+
|
||||
| 1 | 759730a97e4373f3a0ee12805db065e3a4a649a5 | | google |
|
||||
| 2 | de357e976171e528088843dfa35c1097017b1009 | 759730a97e4373f3a0ee12805db065e3a4a649a5 | askjeeves |
|
||||
| 3 | 1b69dff11f3e8ffeade0f42521f9e1bd1bd78539 | de357e976171e528088843dfa35c1097017b1009 | bing |
|
||||
| 4 | 924660e4f25e2ac8c38ca25bae201ad3a5b6e545 | 1b69dff11f3e8ffeade0f42521f9e1bd1bd78539 | altavista |
|
||||
+----+------------------------------------------+------------------------------------------+-----------+
|
||||
```
|
||||
|
||||
As you can see, row 2\'s `url` is updated, and rows 3 and 4 have updated
|
||||
signatures and parents. Pretty cool, and pretty much the same thing as
|
||||
what happens when you change a git commit via `rebase` --- all the
|
||||
successive commits get new SHAs.
|
||||
|
||||
|
||||
[[Learn More]{.util-breadcrumb-md .mb-8 .group-hover:translate-y-20
|
||||
.group-hover:opacity-0 .transition-all .ease-in-out
|
||||
.duration-500}](https://www.viget.com/careers/application-developer/){.relative
|
||||
.flex .group .flex-col .p-32 .md:p-40 .lg:p-64 .z-10}
|
||||
|
||||
### We're hiring Application Developers. Learn more and introduce yourself. {#were-hiring-application-developers.-learn-more-and-introduce-yourself. .text-20 .md:text-24 .lg:text-32 .font-bold .leading-[170%] .group-hover:-translate-y-20 .transition-transform .ease-in-out .duration-500}
|
||||
|
||||
{.rect-icon-md
|
||||
.self-end .mt-16 .group-hover:-translate-y-20 .transition-all
|
||||
.ease-in-out .duration-500}
|
||||
|
||||
I\'ll be honest that I don\'t have any immediately practical uses for a
|
||||
cryptographically-signed database table, but I thought it was cool and
|
||||
helped me understand these concepts a little bit better. Hopefully it
|
||||
gets your mental wheels spinning a little bit, too. Thanks for reading!
|
||||
|
||||
------------------------------------------------------------------------
|
||||
|
||||
1. ::: {#fn:1}
|
||||
[Here\'s a pretty good explanation of what mining really
|
||||
is](https://asthasr.github.io/posts/how-blockchains-work/), but, in
|
||||
a nutshell, it\'s running a hashing algorithm over and over again
|
||||
with a random salt until a hash is found that begins with a required
|
||||
number of zeroes. [ ↩︎](#fnref:1 "return to body"){.reversefootnote}
|
||||
:::
|
||||
Reference in New Issue
Block a user