diff --git a/src/Helpers/HashHelper.php b/src/Helpers/HashHelper.php
index f48676e..74ef78e 100644
--- a/src/Helpers/HashHelper.php
+++ b/src/Helpers/HashHelper.php
@@ -18,23 +18,27 @@ class HashHelper
     ];
 
     /**
-     * Generates the hash for an invoice record.
+     * Generates the hash for an invoice record according to AEAT specifications.
+     * 
+     * CRITICAL: Field names MUST match the official AEAT XML field names as per
+     * "Detalle de las especificaciones técnicas para generación de la huella o hash
+     * de los registros de facturación" v0.1.2 (27/08/2024), page 6.
      *
-     * @param array $data Invoice record data in the correct order.
+     * @param array $data Invoice record data with snake_case keys (for compatibility).
      * @return array ['hash' => string, 'inputString' => string]
      */
     public static function generateInvoiceHash(array $data): array
     {
-        self::validateData(self::$invoiceRequiredFields, $data);
-        $inputString = self::field('issuer_tax_id', $data['issuer_tax_id']);
-        $inputString .= self::field('invoice_number', $data['invoice_number']);
-        $inputString .= self::field('issue_date', $data['issue_date']);
-        $inputString .= self::field('invoice_type', $data['invoice_type']);
-        $inputString .= self::field('total_tax', $data['total_tax']);
-        $inputString .= self::field('total_amount', $data['total_amount']);
-        $inputString .= self::field('previous_hash', $data['previous_hash']);
-        $inputString .= self::field('generated_at', $data['generated_at'], false);
-        $hash = strtoupper(hash('sha256', $inputString, false));
+        self::validateData(self::$invoiceRequiredFields, $data);                
+        $inputString = self::field('IDEmisorFactura', $data['issuer_tax_id']);
+        $inputString .= self::field('NumSerieFactura', $data['invoice_number']);
+        $inputString .= self::field('FechaExpedicionFactura', $data['issue_date']);
+        $inputString .= self::field('TipoFactura', $data['invoice_type']);
+        $inputString .= self::field('CuotaTotal', $data['total_tax']);
+        $inputString .= self::field('ImporteTotal', $data['total_amount']);
+        $inputString .= self::field('Huella', $data['previous_hash']);
+        $inputString .= self::field('FechaHoraHusoGenRegistro', $data['generated_at'], false);                
+        $hash = strtoupper(hash('sha256', $inputString, false));        
         return ['hash' => $hash, 'inputString' => $inputString];
     }
 
diff --git a/tests/Unit/HashHelperAeatComplianceTest.php b/tests/Unit/HashHelperAeatComplianceTest.php
new file mode 100644
index 0000000..064ea54
--- /dev/null
+++ b/tests/Unit/HashHelperAeatComplianceTest.php
@@ -0,0 +1,156 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Squareetlabs\VeriFactu\Tests\Unit;
+
+use PHPUnit\Framework\TestCase;
+use Squareetlabs\VeriFactu\Helpers\HashHelper;
+
+/**
+ * Test compliance with AEAT official specifications for hash generation.
+ * 
+ * Reference: "Detalle de las especificaciones técnicas para generación de la 
+ * huella o hash de los registros de facturación" v0.1.2 (27/08/2024)
+ * 
+ * @see https://sede.agenciatributaria.gob.es/
+ */
+class HashHelperAeatComplianceTest extends TestCase
+{
+    /**
+     * Test Case 1 from AEAT spec (page 10):
+     * First invoice registration in a SIF (no previous hash).
+     * 
+     * Expected hash: 3C464DAF61ACB827C65FDA19F352A4E3BDC2C640E9E9FC4CC058073F38F12F60
+     */
+    public function test_first_invoice_hash_matches_aeat_specification(): void
+    {
+        $data = [
+            'issuer_tax_id' => '89890001K',
+            'invoice_number' => '12345678/G33',
+            'issue_date' => '01-01-2024',
+            'invoice_type' => 'F1',
+            'total_tax' => '12.35',
+            'total_amount' => '123.45',
+            'previous_hash' => '', // Empty for first invoice
+            'generated_at' => '2024-01-01T19:20:30+01:00',
+        ];
+
+        $result = HashHelper::generateInvoiceHash($data);
+
+        // Verify hash format (64 chars, uppercase hex)
+        $this->assertMatchesRegularExpression('/^[A-F0-9]{64}$/', $result['hash']);
+        
+        // Verify hash matches AEAT expected value
+        $this->assertEquals(
+            '3C464DAF61ACB827C65FDA19F352A4E3BDC2C640E9E9FC4CC058073F38F12F60',
+            $result['hash'],
+            'Hash does not match AEAT official example for first invoice'
+        );
+
+        // Verify input string uses official AEAT field names
+        $expectedInputString = 'IDEmisorFactura=89890001K&NumSerieFactura=12345678/G33&'
+            . 'FechaExpedicionFactura=01-01-2024&TipoFactura=F1&CuotaTotal=12.35&'
+            . 'ImporteTotal=123.45&Huella=&FechaHoraHusoGenRegistro=2024-01-01T19:20:30+01:00';
+
+        $this->assertEquals($expectedInputString, $result['inputString']);
+    }
+
+    /**
+     * Test Case 2 from AEAT spec (page 11):
+     * Second invoice with previous hash.
+     * 
+     * Expected hash: F7B94CFD8924EDFF273501B01EE5153E4CE8F259766F88CF6ACB8935802A2B97
+     */
+    public function test_second_invoice_hash_matches_aeat_specification(): void
+    {
+        $data = [
+            'issuer_tax_id' => '89890001K',
+            'invoice_number' => '12345679/G34',
+            'issue_date' => '01-01-2024',
+            'invoice_type' => 'F1',
+            'total_tax' => '12.35',
+            'total_amount' => '123.45',
+            'previous_hash' => '3C464DAF61ACB827C65FDA19F352A4E3BDC2C640E9E9FC4CC058073F38F12F60',
+            'generated_at' => '2024-01-01T19:20:35+01:00',
+        ];
+
+        $result = HashHelper::generateInvoiceHash($data);
+
+        $this->assertEquals(
+            'F7B94CFD8924EDFF273501B01EE5153E4CE8F259766F88CF6ACB8935802A2B97',
+            $result['hash'],
+            'Hash does not match AEAT official example for second invoice'
+        );
+    }
+
+    /**
+     * Test that hash output is always uppercase (page 9 of spec).
+     */
+    public function test_hash_output_is_uppercase(): void
+    {
+        $data = [
+            'issuer_tax_id' => '89890001K',
+            'invoice_number' => 'TEST-001',
+            'issue_date' => '01-01-2024',
+            'invoice_type' => 'F1',
+            'total_tax' => '10.00',
+            'total_amount' => '100.00',
+            'previous_hash' => '',
+            'generated_at' => '2024-01-01T12:00:00+01:00',
+        ];
+
+        $result = HashHelper::generateInvoiceHash($data);
+
+        $this->assertMatchesRegularExpression(
+            '/^[A-F0-9]{64}$/',
+            $result['hash'],
+            'Hash must be 64 uppercase hexadecimal characters'
+        );
+        
+        $this->assertEquals(
+            strtoupper($result['hash']),
+            $result['hash'],
+            'Hash must be in uppercase'
+        );
+    }
+
+    /**
+     * Test whitespace trimming (page 6 of spec):
+     * Values with leading/trailing spaces should be trimmed.
+     */
+    public function test_whitespace_trimming(): void
+    {
+        $dataWithSpaces = [
+            'issuer_tax_id' => '89890001K',
+            'invoice_number' => ' 12345678 / G33 ', // Spaces inside preserved
+            'issue_date' => '01-01-2024',
+            'invoice_type' => 'F1',
+            'total_tax' => '12.35',
+            'total_amount' => '123.45',
+            'previous_hash' => '',
+            'generated_at' => '2024-01-01T19:20:30+01:00',
+        ];
+
+        $dataWithoutSpaces = [
+            'issuer_tax_id' => '89890001K',
+            'invoice_number' => '12345678 / G33', // Trimmed
+            'issue_date' => '01-01-2024',
+            'invoice_type' => 'F1',
+            'total_tax' => '12.35',
+            'total_amount' => '123.45',
+            'previous_hash' => '',
+            'generated_at' => '2024-01-01T19:20:30+01:00',
+        ];
+
+        $hashWithSpaces = HashHelper::generateInvoiceHash($dataWithSpaces)['hash'];
+        $hashWithoutSpaces = HashHelper::generateInvoiceHash($dataWithoutSpaces)['hash'];
+
+        $this->assertEquals(
+            $hashWithSpaces,
+            $hashWithoutSpaces,
+            'Leading/trailing spaces should be trimmed but internal spaces preserved'
+        );
+    }
+}
+